fix(pydantic): decode escaped local JSON schema references

[weivwang]

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Changes being requested

Decode URI fragments and RFC 6901 escape sequences while resolving local JSON Schema references in the handwritten Pydantic helper.

resolve_ref() currently splits a local $ref and uses each raw segment as a dictionary key. Definitions containing /, ~, spaces, or other escaped characters therefore fail with KeyError, for example:

{"$ref":"#/$defs/path~1to%20model~0v1"}

The change decodes each segment after splitting it, then applies JSON Pointer's ~1 → / and ~0 → ~ substitutions. Decoding after the split is important: a percent-encoded / belongs to the definition key and must not become an extra path segment.

Two regressions cover both direct resolution and the real strict-schema inlining path with sibling properties.

Additional context & links

No matching issue or open PR was found for escaped local $ref handling. The modified source is under src/openai/lib/, which CONTRIBUTING.md identifies as outside the generated SDK surface.

Validation:

• pytest -q tests/lib/test_pydantic.py — 5 passed
• ruff check src/openai/lib/_pydantic.py tests/lib/test_pydantic.py
• ruff format --check src/openai/lib/_pydantic.py tests/lib/test_pydantic.py
• git diff --check