fix(google): keep tool argument defaults in the Gemini schema

[Darshak03]

_GeminiJsonSchema._simplify dropped default from every property of a tool's JSON Schema:

schema.pop("title", None)
schema.pop("default", None)   # removed
schema.pop("additionalProperties", None)

google.genai.types.Schema has a default field, so unlike title or $schema this is not compatibility stripping — it removes information Gemini accepts and uses. For an argument the caller pre-filled, that default is the model's only record of the value: with it gone the model omits the argument, and a tool that needs it (an MCP server expecting automationId / version, say) rejects the call with an invalid-arguments error.

Both paths through _GeminiJsonSchema are affected — the chat() LLM path, and the realtime path, which is routed through simplify() for raw-schema function tools because Gemini Live does not accept parameters_json_schema (#5560, googleapis/python-genai#1147).

Tests: test_default_is_kept asserts the defaults survive and round-trips the result through types.Schema, so the kept value is proven acceptable to the genai type. Two existing expectations are updated to keep the defaults they previously asserted were stripped.

A nullable field (Location | None = None) now keeps "default": None. That is left as-is rather than filtered: it is the schema as declared, and it serializes away.

schema.pop("default", None) has been there since #1588 and looks carried over from the pydantic-ai implementation this class is based on, alongside the keywords Gemini genuinely rejects.

Fixes #7189

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.