feat: allowlisting MCP tools for generate_content sessions

[a2105z]

Summary

• Fixes #1089: opt-in allowlist so tools=[mcp_session] does not always expand every tool from list_tools().
• Adds google.genai.mcp.with_allowed_tools(session, [...]), which wraps the session, filters list_tools(), and rejects disallowed call_tool() names.
• Bare ClientSession behavior is unchanged. Filtering happens before adapter conversion so function declarations and the AFC map stay aligned for both generate_content and Live.

Usage

from google.genai import mcp as genai_mcp

config = types.GenerateContentConfig(
    tools=[genai_mcp.with_allowed_tools(session, ['tool_a', 'tool_b'])],
)

Test plan

• pytest google/genai/tests/mcp/ -q
• Coverage for allowlist filter, unknown-name, empty-allowlist, and call_tool rejection
• Bare MCP session still exposes all tools (opt-in wrapper only)
• Confirm docs/examples for mcp.with_allowed_tools(session, [...]) look clear in review

[a2105z]

@Venkaiahbabuneelam hey — opened this for #1089.

Passing an MCP session in tools= currently expands every tool from list_tools() into the model schemas + AFC map. allowed_function_names only limits what the model can pick on the wire, so unused tools still show up, which isnt really what people want here.

Went with an opt-in wrapper instead so bare sessions stay the same:

from google.genai import mcp as genai_mcp

tools=[genai_mcp.with_allowed_tools(session, ["tool_a", "tool_b"])]

Filters in list_tools() and blocks disallowed call_tool() calls, so the declarations and adapter map dont get out of sync. Happy to tweak the API if youd rather it live somewhere else.

[a2105z]

@Venkaiahbabuneelam quick follow-up — mypy was failing because is_mcp_client_session() didnt narrow the union, so tool.list_tools() looked unsafe. Pushed a fix that makes that check a TypeGuard and lets the adapter accept the allowlist wrapper. Local mypy (same as CI) is clean now (Success: no issues found in 36 source files). Looks like the new workflow run is waiting on approval to start — whenever you can re-run / approve checks thatd be great.

[a2105z]

@Venkaiahbabuneelam sorry about the mypy noise on this one — that was on me. I missed that the session check wasnt narrowing the type, so CI blew up on list_tools().

Just pushed a fix for it. If you get a sec, could you re-approve / re-run the checks? Appreciate it.