feat: stamp gateway attribution metadata

[haasonsaas]

Summary

• add team, workload, session, and evidence attribution metadata to the Google Generative AI shim
• pass the new attribution options through configure() into default metadata
• keep org/principal/trace behavior backward compatible

Tests

• .venv/bin/python -m pytest tests

[cursor]

PR Summary

Low Risk
Low risk: additive metadata/header stamping only, with tests updated; behavior should remain backward compatible unless downstream systems are sensitive to new headers.

Overview
Adds optional attribution fields (team_id, workload, session_id, evidence_id) to build_evalops_metadata, sourcing from args or EVALOPS_* env vars and emitting corresponding x-evalops-* headers.

Plumbs the same options through configure() so they are included in the Google Generative AI client default_metadata, and updates tests to assert the new headers are stamped.

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