fix(interactions): avoid duplicate User-Agent headers

[MukundaKatta]

Summary

• make library version header injection reuse any existing User-Agent key case-insensitively
• default to a single title-case User-Agent entry when injecting telemetry headers
• add an interactions regression test that verifies only one user-agent value is emitted

Testing

• python3 -m py_compile google/genai/_api_client.py google/genai/tests/interactions/test_integration.py
• python3 -m pytest google/genai/tests/interactions/test_integration.py -k user_agent (fails locally: ModuleNotFoundError: No module named 'google.auth')