feat(python): upgrade to IR v66

[Swimburger]

Description

Implements full IR v66 support for the Python SDK, Pydantic, and FastAPI generators. IR v66 compresses the Name type into a NameOrString union (plain string or full Name object), reducing IR wire size and improving performance.

This PR adds CaseConverter and getWireValue/getOriginalName helpers throughout the Python generators so they can transparently handle both compressed (string) and full (Name/NameAndWireValue) forms.

Changes Made

Generator code updates

• AbstractPythonGeneratorContext: Added CaseConverter instance; replaced direct .snakeCase.unsafeName, .pascalCase.safeName, .snakeCase.safeName accesses with caseConverter.snakeUnsafe(), caseConverter.pascalSafe(), caseConverter.snakeSafe()
• DynamicSnippetsGeneratorContext: Same CaseConverter pattern for getClassName, getPropertyName, getMethodName, getEnvironmentEnumName, and fernFilepath.allParts mapping
• DynamicTypeLiteralMapper: Replaced .wireValue direct accesses with getWireValue() for discriminant values, property names, and enum values
• EndpointSnippetGenerator: Replaced .wireValue with getWireValue() for auth parameter and header lookups
• FilePropertyMapper: Added getWireValue() for file property wire value fallbacks
• ReadmeSnippetBuilder: CaseConverter for environment names, auth scheme names, service paths, endpoint names; null-safety guards for environment name access
• buildReference.ts: CaseConverter for endpoint names, service paths, parameter names, type names, section titles; null-safety guards for environment names
• WireTestGenerator: CaseConverter for service/endpoint names, getWireValue() for query params, headers, path params, and file upload keys; getOriginalName() for path parameter names
• WireTestSetupGenerator: CaseConverter for base URL names, auth scheme names, global header names; getWireValue() for query parameter datetime detection
• EnumGenerator: CaseConverter for enum member names (screamingSnakeSafe, snakeSafe); getWireValue() for enum wire values
• ObjectGenerator: getWireValue() for property wire value comparison and alias detection
• WrappedAliasGenerator: CaseConverter for named type visitor method names (snakeUnsafe)

Version & config updates

• Added version entries in versions.yml for SDK (5.3.0-rc.0), Pydantic (1.12.0-rc.0), FastAPI (2.2.0-rc.0) with irVersion: 66
• Updated migrateFromV66ToV65.ts to register all three generators
• Updated seed.yml irVersion to v66 for python-sdk, pydantic, pydantic-v2, and fastapi

Test fix

• test_ir_deserialization.py: Pinned --version v65 when generating IR via the CLI, because the published CLI now produces v66 IR with compressed names that the Python IR SDK (fern_fern_ir_v65 Pydantic models) cannot deserialize. This is a targeted fix — when a v66 Python IR SDK is published, this version pin should be updated to match.

Testing

• All seed tests pass (python-sdk, pydantic, fastapi)
• All required CI checks pass (306 pass, 0 fail)
• Manual testing completed

Human Review Checklist

• test_ir_deserialization.py version pin: The test now passes --version v65 to generate IR the Python SDK can parse. This is correct for now but creates a maintenance item — when fern_fern_ir_v66 is published, this pin and the pyproject.toml dependency should both be updated.
• CaseConverter initialized with keywords: undefined in multiple files — confirm this is intentional for Python generators (the TypeScript reference impl also passes undefined for keywords in some cases)
• Null-safety in ReadmeSnippetBuilder and buildReference: Environment name access now uses if (defaultEnv?.name != null) guard before calling caseConverter.screamingSnakeUnsafe() — this changes prior behavior where defaultEnv?.name.screamingSnakeCase.unsafeName would have thrown on a compressed string. Verify the guard logic is correct.
• Multiple module-level caseConverter instances with identical config across files — not a bug, but could be centralized if desired

Link to Devin session: https://app.devin.ai/sessions/1947204928ba4bfe9b663dfe828ac3d7
Requested by: @Swimburger

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[devin-ai-integration]

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[graphite-app]

Import inconsistency detected. This file imports getWireValue from @fern-api/base-generator, but all other files in the dynamic-snippets module import from @fern-api/browser-compatible-base-generator (see EndpointSnippetGenerator.ts, DynamicSnippetsGeneratorContext.ts, and DynamicTypeLiteralMapper.ts). This will cause a runtime import error if getWireValue is not exported from @fern-api/base-generator or if the browser-compatible variant is required.

Fix:

import { getWireValue } from "@fern-api/browser-compatible-base-generator";

Suggested change

	import { getWireValue } from "@fern-api/base-generator";
	import { getWireValue } from "@fern-api/browser-compatible-base-generator";

Spotted by Graphite



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[devin-ai-integration]

Devin Review found 1 new potential issue.

View 11 additional findings in Devin Review.

[devin-ai-integration]

🟡 FilePropertyMapper imports getWireValue from non-browser-compatible package

In FilePropertyMapper.ts, getWireValue is imported from @fern-api/base-generator, while all other files in the same dynamic-snippets package (DynamicTypeLiteralMapper.ts, EndpointSnippetGenerator.ts, DynamicSnippetsGeneratorContext.ts) import from @fern-api/browser-compatible-base-generator. The dynamic-snippets package is designed to be browser-compatible, and importing from @fern-api/base-generator could introduce Node.js-specific dependencies into the browser bundle, potentially breaking browser builds or bloating bundle size.

Suggested change

	import { assertNever } from "@fern-api/core-utils";
	import { getWireValue } from "@fern-api/browser-compatible-base-generator";

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[devin-ai-integration]

Acknowledged — however @fern-api/browser-compatible-base-generator doesn't currently export getWireValue. The dynamic-snippets package.json lists @fern-api/base-generator as a dependency, so the import works. If browser compatibility is a concern, getWireValue would need to be re-exported from the browser-compatible package first.

[github-actions]

SDK Generation Benchmark Results

Comparing PR branch against main baseline.

Full benchmark table (click to expand)

Generator	Spec	main (generator)	main (E2E)	PR (generator)	Delta
python-sdk	square	N/A	N/A	168s	N/A

main (generator): generator-only time via --skip-scripts (includes Docker image build, container startup, IR parsing, and code generation — this is the same Docker-based flow customers use via fern generate). main (E2E): full customer-observable time including build/test scripts (nightly baseline, informational). Delta is computed against generator-only baseline.
⚠️ = generation exited with a non-zero exit code (timing may not reflect a successful run).

[github-actions]

SDK Generation Benchmark Results

Comparing PR branch against main baseline.

Full benchmark table (click to expand)

Generator	Spec	main (generator)	main (E2E)	PR (generator)	Delta
python-sdk	square	N/A	N/A	167s	N/A

main (generator): generator-only time via --skip-scripts (includes Docker image build, container startup, IR parsing, and code generation — this is the same Docker-based flow customers use via fern generate). main (E2E): full customer-observable time including build/test scripts (nightly baseline, informational). Delta is computed against generator-only baseline.
⚠️ = generation exited with a non-zero exit code (timing may not reflect a successful run).