fix(core): preserve reasoning metadata on errored assistant turns

[loulanyue]

Fixes #38620.

Anthropic rejects a replayed tool_use with a 400 if its preceding thinking block is dropped when extended thinking is enabled.

Previously, reuseProviderMetadata required message.error === undefined, meaning an aborted/failed step would drop all reasoning state on replay (both anthropic.signature and anthropic.redactedData) while still replaying that same message's tool_use parts.

This relaxes the guard strictly for reasoning blocks, ensuring the signature is retained when sameModel is true. This keeps the block from being filtered out as empty, keeping the message structurally valid for the provider.

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

Based on my search, I found one potentially related PR that may be addressing similar concerns about reasoning metadata preservation:

Related PR:

• fix: preserve thinking/redacted_thinking blocks in Anthropic message transforms #23755 - fix: preserve thinking/redacted_thinking blocks in Anthropic message transforms
  • This PR deals with preserving thinking blocks in Anthropic message transforms, which is related to the reasoning metadata handling in the current PR (fix(core): preserve reasoning metadata on errored assistant turns #38711).

However, this appears to be an older PR and addresses a different aspect of the problem. The current PR (#38711) is specifically fixing the issue where reasoning metadata is dropped on errored/aborted assistant turns during replay, which is a distinct bug from preserving thinking blocks in general transforms.

No duplicate PRs found

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