RSPEED-3221: round-trip Gemini 3 thought signatures through Vertex AI converter

[major]

Description

Gemini 3.x models (e.g. gemini-3-flash, gemini-3.5-flash) attach a thought_signature to the first functionCall part of a tool-calling turn and require it to be replayed verbatim on the next turn, or the request fails with HTTP 400. llama-stack converts Gemini responses into the OpenAI chat-completion shape, which has no field for the signature, so it gets dropped and every multi-turn tool call against a Gemini 3 model fails.

This monkeypatches llama-stack's Vertex AI converter at app import time. Both wrappers defer entirely to the upstream originals and only smuggle the base64-encoded signature in and out through the opaque tool-call id (which llama-stack round-trips untouched and only ever compares for equality):

• The extract wrapper re-pairs each functionCall part with the tool call the original emitted and embeds the signature in its id.
• The assistant-message wrapper decodes it back onto the rebuilt Gemini part.

The patch is idempotent and a no-op when the Vertex AI provider is not installed. This is a downstream shim; remove it once the fix lands upstream in llama-stack.

Type of change

• Bug fix

Tools used to create PR

• Assisted-by: Claude
• Generated by: N/A

Related Tickets & Documents

• Related Issue # RSPEED-3221

Checklist before requesting a review

• I have performed a self-review of my code.
• PR has passed all pre-merge test jobs.
• If it is a core feature, I have added thorough tests.

Testing

Unit tests cover the encode/decode helpers, idempotency of the patch, the no-op path when the Vertex AI provider is absent, signature embedding into the tool-call id, and the full extract -> assistant-message round trip.

uv run pytest tests/unit/utils/test_vertexai_thought_signature.py -q
# 10 passed

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