Add server default result storage config surface

[zzstoatzz]

this PR adds the OSS config surface for a server-level default result storage setting.

What changed

• adds a ServerDefaultResultStorage schema for representing the configured block document id
• adds /admin/storage read, update, and clear endpoints
• adds client methods for reading, updating, and clearing the server default result storage setting
• validates that the configured block exists and supports write-path
• keeps storage-default-specific validation and persistence in a dedicated server.models.storage_defaults module instead of leaving that logic inline in the route
• syncs the generated ui-v2 OpenAPI client for the new admin surface

Why this is a separate slice

This PR only adds the configuration surface for storing a server default result storage block. It does not change runtime result-storage resolution yet.

That separation keeps the persisted setting and API contract reviewable on their own before the follow-up runtime behavior work lands.

Verification

• uv run pytest tests/server/api/test_admin.py tests/server/models/test_storage_defaults.py tests/client/test_prefect_client.py -k 'server_default_result_storage'
• uv run pytest tests/server/models/test_configuration.py tests/server/models/test_storage_defaults.py
• uv run ruff check src/prefect/server/api/admin.py src/prefect/server/models/__init__.py src/prefect/server/models/configuration.py src/prefect/server/models/storage_defaults.py tests/server/api/test_admin.py tests/server/models/test_configuration.py tests/server/models/test_storage_defaults.py tests/client/test_prefect_client.py
• repo hooks passed during commit and push, including mypy and Sync UI v2 OpenAPI

[codspeed-hq]

Merging this PR will not alter performance

✅ 2 untouched benchmarks

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_{Comparing codex/storage-default-config-surface (78a9f1a) with main (b89b43d)}

[github-actions]

This pull request is stale because it has been open 14 days with no activity. To keep this pull request open remove stale label or comment.

[github-actions]

This pull request was closed because it has been stale for 14 days with no activity. If this pull request is important or you have more to add feel free to re-open it.

[zzstoatzz]

Superseded by #21532 after rebasing codex/storage-default-config-surface onto current main and refreshing the PR description. Keeping this closed PR as historical reference.