fix: handle boolean mcp schema values

[MukundaKatta]

Fixes #2393.

Summary

• make MCP schema filtering tolerate non-dict schema values such as boolean additionalProperties
• add regression coverage for FastMCP-style schemas with additionalProperties: false

Tests

• uv run --with pytest --with pytest-asyncio --with mcp pytest google/genai/tests/mcp/test_mcp_to_gemini_tools.py -q
• git diff --check