fix(client): reset aiohttp session before mTLS retry

[MukundaKatta]

Summary

• close and clear the existing aiohttp session before retrying connector failures in the async mTLS path
• ensure the retry path creates a fresh AsyncAuthorizedSession instead of reusing the broken one
• add a regression test that verifies the first mTLS session is closed and replaced on retry

Testing

• python3 -m py_compile google/genai/_api_client.py google/genai/tests/client/test_retries.py
• python3 -m pytest google/genai/tests/client/test_retries.py -k "mtls_session or connector_error_recreates_mtls_session" (the targeted test was skipped in this environment because aiohttp-specific test prerequisites are unavailable)