fix(openai): avoid orphaned Responses item ids

[d3f4w2]

Description

Fixes #12935.

The OpenAI adapter reconstructs Responses API input from Chat Completions history. That history does not contain the separate Responses reasoning item or output-item identity, but the converter was inventing msg_* IDs for assistant messages and treating fc_* tool-call correlation IDs as Responses output-item IDs. On the following tool turn, the Responses API can therefore require a reasoning item that the reconstructed history cannot provide.

This change:

• replays historical assistant content as an easy input message without a synthetic msg_* output-item ID;
• keeps Chat Completions tool-call IDs only as call_id, instead of also treating fc_* values as Responses item IDs;
• adds focused regression coverage for assistant history and a complete assistant tool-call/tool-result turn.

This is separate from the core converter's context-compaction sanitizer: this path is the standalone packages/openai-adapters Chat Completions-to-Responses adapter, where reasoning/output-item metadata is unavailable by construction.

AI assistance disclosure: Codex assisted with investigation, implementation, and tests; I reviewed the resulting diff and validation results.

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Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created (not applicable; no user-facing behavior/configuration change)
• The relevant tests, if any, have been updated or created

Screen recording or screenshot

Not applicable: this fixes request serialization and is covered by adapter regression tests.

Tests

• cd packages/openai-adapters && npm test -- --run — 145 passed, 5 skipped
• cd packages/openai-adapters && npm run build
• npx prettier --check packages/openai-adapters/src/apis/openaiResponses.ts packages/openai-adapters/src/test/openai-responses.vitest.ts
• git diff --check

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