fix(java): use client-class-name for exception type in README

[jsklan]

Description

When client-class-name is specified in the Java generator custom config, the v1 generator creates an exception class named {clientClassName}ApiException. However, the v2 README generator was ignoring client-class-name and always using {org}ApiException (via getBaseNamePrefix()), causing the README's exception handling snippet to reference a class that doesn't exist in the SDK.

Changes Made

• Updated getApiExceptionClassName() in SdkGeneratorContext.ts to check client-class-name before falling back to getBaseNamePrefix(), matching v1's logic in ClientPoetClassNameFactory.getApiErrorClassName()
• Added changelog entry (version 4.0.5)

Priority chain is now:

1. base-api-exception-class-name (explicit override) — unchanged
2. client-class-name + "ApiException" — new
3. getBaseNamePrefix() + "ApiException" (org-based default) — unchanged

Human Review Checklist

• Verify the fallback priority matches v1's ClientPoetClassNameFactory.getApiErrorClassName() at generators/java/sdk/src/main/java/com/fern/java/client/ClientPoetClassNameFactory.java:160-167
• Consider whether similar client-class-name awareness is missing in other v2 naming methods (e.g., getBaseExceptionClassName equivalent if one exists)

Testing

• Unit tests added/updated
• Verified pre-existing compile errors are unrelated to this change
• Manual testing completed

Link to Devin session: https://app.devin.ai/sessions/3cb0ef910a644cf7b635abc52e59943b
Requested by: @jsklan

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