fix(opencode): preserve reasoning providerMetadata across model switches

[bainos]

Fixes #22813

What

Always preserve providerMetadata on reasoning parts when reconstructing conversation history, regardless of the differentModel flag.

Why

The differentModel guard exists to prevent provider-specific metadata (e.g. OpenAI itemId) from leaking into other providers — correct for text and tool parts. But Anthropic thinking blocks carry a cryptographic signature inside providerMetadata that Anthropic verifies on every subsequent turn. Stripping it causes the API to reject the message with "thinking blocks cannot be modified".

How I verified it

• Added a regression test: "preserves reasoning providerMetadata even when assistant model differs" in test/session/message-v2.test.ts
• Full test suite passes (1880/1882; the 2 failures are pre-existing and unrelated)
• Typecheck clean
• Manual testing in progress

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found several related PRs that address similar issues with reasoning/thinking blocks and providerMetadata:

Potentially Related PRs:

1. fix: replace empty text in reasoning messages to preserve thinking block positions #21860 - "fix: replace empty text in reasoning messages to preserve thinking block positions"

   • Related to preserving thinking block structure

2. fix(provider): preserve assistant message content when reasoning blocks present #21370 - "fix(provider): preserve assistant message content when reasoning blocks present"

   • Directly addresses preserving content when reasoning blocks are involved

3. fix: preserve thinking block signatures and fix compaction headroom asymmetry #14393 - "fix: preserve thinking block signatures and fix compaction headroom asymmetry"

   • Addresses thinking block signature preservation (similar concern to your cryptographic signature issue)

4. fix(provider): skip empty-text filtering for assistant messages in normalizeMessages (#16748) #16750 - "fix(provider): skip empty-text filtering for assistant messages in normalizeMessages"

   • Related to message normalization and thinking blocks

These PRs all address similar concerns around preserving thinking block integrity and metadata. However, PR #22818 (the current PR) appears to be the specific fix for issue #22813, so it's likely a unique solution rather than a duplicate of the older PRs.

[github-actions]

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