fix(agent): consume Gemini replay metadata once

[kojiwakayama]

Summary

Follow-up to #3820 and veryfront/veryfront-issue-inbox#549. The internal WeakMap replay metadata added by #3820 was read without being consumed, so a signature from tool step 1 could be replayed again on tool step 3 alongside the current signature. Gemini rejects that stale cross-step replay.

This change makes provider-request replay metadata single-use while preserving ordinary/history conversion behavior. Metadata is consumed only for assistant runtime messages that remain in the final outbound prompt; a trailing assistant message trimmed from the current request retains its metadata for the later request that actually sends it.

RED → GREEN

• RED: a three-request generate flow replayed [signature1, signature2] on request 3 instead of only [signature2].
• RED: consuming before trailing-assistant trimming lost metadata that needed to be sent after the subsequent tool result.
• GREEN: generate, legacy stream, and active stream lifecycles now send only the current step signature.
• GREEN: ordinary/history conversion remains repeatable and non-consuming; provider request conversion consumes only retained outbound metadata.

Verification

• Full pre-push gate: 3,922 tests / 29,978 steps, 0 failed; cwd suites 13 tests / 213 steps, 0 failed.
• Focused provider/runtime suites: 110 steps green.
• Targeted deno check, lint, format, diff check, anti-slop, test-typecheck, public/error docs, and pinned Deno 2.7.7 API docs checks green.
• Independent exact-diff review: approve, zero findings.

Fixes veryfront/veryfront-issue-inbox#549

[kojiwakayama]

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📦 Client bundle boundary

Entrypoint	Modules	Source size	Server leaks
src/index.client.ts	325	1937 KiB	✅ 0

A server module in a client graph aborts hydration in the browser. New leaks fail CI; known leaks are tracked in scripts/lint/client-bundle-baseline.json to burn down.

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📥 Commits

Reviewing files that changed from the base of the PR and between e297edb and 0867dbe.

📒 Files selected for processing (4)

• src/agent/runtime/provider-metadata-continuation.test.ts
• src/agent/runtime/provider-metadata.ts
• src/agent/runtime/text-generation-runtime-message-converter.test.ts
• src/agent/runtime/text-generation-runtime-message-converter.ts

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[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Delightful!

Reviewed commit: 0867dbe1c3

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[kojiwakayama]

Closing as invalid after main merged the authoritative regression in #3821. Google’s Gemini 3 protocol requires every accumulated function-call thought signature in the current sequential tool turn: request 3 must contain signature A and signature B. This PR’s consume-once behavior removes signature A and would restore the provider 400. The late review suggestion on #3820 is therefore rejected on protocol grounds, not implemented.

Primary source: https://ai.google.dev/gemini-api/docs/generate-content/thought-signatures#sequential_function_calling_example