fix(genai): restore accurate Gemini token usage reporting

[parkerhancock]

Why this matters

Teams running Gemini and Vertex adapters today cannot trust the billable token numbers they receive. LangSmith traces, quota safeguards, and customer invoices are all deriving cost from incorrect totals because we drop cache/tool usage, lose modality details, and compute negative or inflated deltas during streaming. Several production users (issues #975, #940, #1011, #1053, #879) have reported that this blocks them from rolling out Gemini 2.x and multimodal workloads; this PR resolves those regressions end to end.

What’s included

• Normalize Gemini usage_metadata so totals always equal inputs + outputs and we preserve cache/tool/modality detail dictionaries across both sync and streaming responses.
• Apply the same normalization to the Vertex adapter (mirroring merged PR fix: Add cache token support to ChatAnthropicVertex streaming responses #1010 for Anthropic) so input_token_details and modality enums flow through exactly as Google returns them.
• Introduce a shared delta helper for streaming that prevents negative/duplicated counts and ensures tool-call prompts and reasoning tokens are visible chunk-by-chunk. Also harden tool-call argument coercion to avoid Cloud Build failures from Infinity/NaN payloads.

Issues addressed

Fixes #975, fixes #940, fixes #1011, fixes #1053, fixes #879

Testing

• GOOGLE_API_KEY=fake uv run pytest libs/genai/tests/unit_tests/test_chat_models.py
• GOOGLE_API_KEY=fake VERTEXAI_LOCATION=us-central1 uv run pytest libs/vertexai/tests/unit_tests/test_usage_metadata.py

[mdrxy]

Thank you - will investigate as soon as able

[parkerhancock]

Updated the branch to latest main, fixed Gemini tool-call argument serialization so Infinity/NaN responses no longer break parsing, and cleaned up the local lint (mypy) failure. The failing Cloud Build run was caused by proto tool-call args containing Infinity; the new _coerce_function_call_args path normalizes those values and we added a regression test to cover it. Lint passes with make lint locally; Google Cloud Build should rerun clean now.