Python (fix:gemini): make Gemini honor declarative outputSchema, not just JSON mode (#5893)

* fix(gemini): preserve schema response_format

* fix(gemini): satisfy pyright strict in response schema extraction

Cast Any-narrowed mappings to Mapping[str, Any] in the structured-output
schema helpers so pyright strict no longer reports partially-unknown
member, argument, and variable types. Pass response_format["format"]
straight into the recursive extractor, which already guards non-mapping
inputs. No behavior change.

* fix(gemini): use Sequence[object] cast to satisfy both mypy and pyright

The Sequence[Any] cast pyright strict needs to know the loop element type
is reported as a redundant-cast by mypy, which already narrows the
isinstance branch to Sequence[Any]. Cast to Sequence[object] instead:
pyright gets a fully known element type and mypy no longer sees an
identical-type cast. No behavior change.

---------

Co-authored-by: Evan Mattson <evan.mattson@microsoft.com>

python/packages/gemini/README.md

@@ -10,6 +10,10 @@ pip install agent-framework-gemini --pre
 
 The Gemini integration enables Microsoft Agent Framework applications to call Google Gemini models with familiar chat abstractions, including streaming, tool/function calling, and structured output.
 
+## Structured Output
+
+Gemini structured output can be configured with either a Pydantic model in `response_format`, a JSON schema mapping in `response_format`, or a Gemini-specific `response_schema`. Declarative agents that define `outputSchema` pass that schema through `response_format`.
+
 ## Authentication
 
 The connector supports both `google-genai` authentication modes.