refactor(py): rename OutputConfig.schema_ to json_schema, pin wire alias

[huangjeff5]

Product Context & Motivation

In the Genkit Python SDK, structured output configuration had inconsistent field naming: GenerateActionOutputConfig used json_schema, whereas OutputConfig used schema_ to avoid shadowing Pydantic's BaseModel.schema. This inconsistency required linter warning suppressions (filterwarnings) and created friction for developers interacting with output schemas in Python code.

This PR renames OutputConfig.schema_ to json_schema in Python while pinning the serialization alias to schema. Developers get a clean, consistent Python API (config.json_schema), while Genkit's cross-language wire protocol (Dev UI, reflection server, TS/Go SDKs) continues to send and receive "schema" with 100% wire fidelity.

Key Changes

• Renamed Python field OutputConfig.schema_ to json_schema.
• Configured Pydantic field aliases: validation_alias=AliasChoices('json_schema', 'schema') (accepts both input names) and serialization_alias='schema' (serializes wire key as "schema").
• Updated code generator (schema_to_typing.py) to emit the new alias configuration cleanly.
• Removed legacy warnings.filterwarnings hacks.

Verification & Safety

• 100% backward compatible: input validation accepts both json_schema and schema. Wire payloads remain identical.
• Verified with full core suite pytest py/packages/genkit/tests/genkit/ (799 passed).

[gemini-code-assist]

Code Review

This pull request refactors the OutputConfig model to use json_schema instead of schema_ to avoid shadowing the parent BaseModel.schema attribute. It leverages Pydantic's AliasChoices for validation and serialization aliases to maintain compatibility with the wire format, allowing the removal of warning filters. The schema generation script schema_to_typing.py has been updated accordingly. There are no review comments, so no additional feedback is provided.