Python: fix(gemini): forward response_format as response_schema (#5888)

[Protocol-zero-0]

Summary

Fixes #5888 — when a declarative agent's `outputSchema` reaches the Gemini chat client, the schema was dropped and the model returned arbitrary JSON instead of conforming to the schema.

Root cause

The Gemini chat client only forwarded the schema to the underlying Google GenAI `response_schema` field when the user used Gemini's own `response_schema` option. The cross-client `response_format` option (which is what `AgentFactory` from `agent_framework_declarative` populates from `outputSchema`) only triggered `response_mime_type=application/json` but left `response_schema` unset, so Gemini received JSON mode with no schema.

`python/packages/gemini/agent_framework_gemini/_chat_client.py` previously:

```python
if options.get("response_format") or options.get("response_schema"):
kwargs["response_mime_type"] = "application/json"
if schema := options.get("response_schema"):
kwargs["response_schema"] = schema
```

Fix

When `response_schema` is not explicitly set, fall back to `response_format` and forward it. Handles both raw JSON schema dicts (what the declarative loader produces) and Pydantic `BaseModel` classes. Explicit `response_schema` continues to take precedence.

Tests

Added three regression tests in `packages/gemini/tests/test_gemini_client.py`:

• `test_response_format_dict_forwarded_as_response_schema` — the issue-5888 path
• `test_response_format_pydantic_forwarded_as_response_schema` — symmetry for the Pydantic case
• `test_response_schema_takes_precedence_over_response_format` — pins the precedence rule

```
99 passed, 8 skipped
```

`ruff check` and `ruff format --check` clean on the touched files.

Scope

Surgical change inside `_build_generate_content_config`. No public API surface change; existing tests untouched.

Fixes #5888

[Copilot]

Pull request overview

Fixes a bug where the Gemini chat client dropped response_format schemas (used by declarative agents' outputSchema), leaving the model in JSON mode without a schema. The fix forwards response_format to Gemini's native response_schema field when the explicit response_schema option is not set.

Changes:

• In _prepare_config, fall back to response_format when response_schema is unset; supports both raw JSON schema dicts and Pydantic BaseModel subclasses.
• Explicit response_schema continues to take precedence.
• Added three regression tests covering dict, Pydantic, and precedence cases.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
python/packages/gemini/agent_framework_gemini/_chat_client.py	Forward response_format to Gemini's response_schema when no explicit schema is set.
python/packages/gemini/tests/test_gemini_client.py	Regression tests for dict/Pydantic forwarding and explicit-schema precedence.