fix: refresh expired OAuth tokens for older chat sessions

[ShivamRIG]

Fixes #5556

Problem:
Old chat sessions retained expired access tokens in session.state.
Tokens were never refreshed, causing tools to fail in older chats.
Reloading the page did not help as the session is server-side.

Fix:
Added _refresh_token_if_expired() method to A2aAgentExecutor.
Called after _prepare_session() in _handle_request(), before
the runner executes. Checks expiry, refreshes using refresh_token,
and updates session state with new access_token and expires_at.

Tests:
Added 3 unit tests covering:

• Valid token → no refresh triggered
• Expired token → refreshed correctly
• Missing refresh_token → skips silently

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[ShivamRIG]

Testing Plan

Added 3 unit tests in tests/unittests/a2a/executor/test_refresh_token.py:

• test_token_not_expired_skips_refresh — valid token, no refresh triggered
• test_expired_token_triggers_refresh — expired token refreshed correctly
• test_no_refresh_token_skips_refresh — missing refresh_token skips silently

All 3 tests pass:
pytest tests/unittests/a2a/executor/test_refresh_token.py -v
3 passed in 0.XXs

[rohityan]

Hi @ShivamRIG ,Thank you for your contribution! It appears you haven't yet signed the Contributor License Agreement (CLA). Please visit https://cla.developers.google.com/ to complete the signing process. Once the CLA is signed, we'll be able to proceed with the review of your PR. Thank you!