Issue

Closes #6938.

Summary

MCP 2.x expects timeout values as float seconds, but _read_timeout() always returned datetime.timedelta, causing anyio.fail_after() to fail during MCP session initialization. This keeps the existing MCP 1.x conversion and returns the original float for MCP 2.x.

Testing plan

• python -m compileall -q src/google/adk/tools/mcp_tool/session_context.py tests/unittests/tools/mcp_tool/test_session_context.py — passed
• git diff --check — passed
• Focused pytest — not run: the host has Pydantic 1.x while ADK requires Pydantic 2.x, so collection fails before tests execute.
• Ruff on the two touched files — reports two pre-existing unused imports in test_session_context.py (AsyncExitStack and ClientSession); no new lint findings were introduced.

The regression tests cover MCP 1.x timedelta behavior, MCP 2.x float behavior, None, zero, and fractional values. No external service is required.