V2: errored-message replay can 400 with Anthropic thinking + tool use

Problem

In the V2 session core, replaying history that contains an errored assistant message can produce a request Anthropic rejects with a 400 when extended thinking is enabled.

reuseProviderMetadata requires message.error === undefined (packages/core/src/session/runner/to-llm-message.ts:114), so an aborted/failed step drops all reasoning state on replay — both signed thinking (anthropic.signature) and redacted thinking (anthropic.redactedData) — while still replaying that same message's tool_use parts.

Anthropic requires a tool_use block to be preceded by its thinking/redacted_thinking block when thinking is enabled. Replaying tool_use without the reasoning block yields a 400 from the API.

Impact

Multi-turn thinking + tool-use conversations with Anthropic break after an interrupt or failure mid-step: the follow-up request replays the errored message's tool calls stripped of their reasoning blocks and the provider rejects it.

Scope

• Pre-existing behavior; unrelated to and not introduced by #38614 (which added redacted thinking round-trip support in packages/ai). Signed thinking is affected identically.
• The errored-message guard is otherwise a deliberate design decision, so the fix should be deliberate too.

Possible directions

• Relax the message.error === undefined guard in to-llm-message.ts so reasoning provider state is reused for errored messages when provider+model match, or
• Drop the errored message's tool-call parts alongside the reasoning so the replayed message stays self-consistent (no orphaned tool_use).

Either way, the invariant to preserve is: never replay a tool_use part to Anthropic without its preceding thinking block while thinking is enabled.

Start in packages/core/src/session/runner/to-llm-message.ts around line 114 and trace how reuseProviderMetadata handles errored messages during V2 history replay. Compare the two proposed directions and verify the resulting replay preserves Anthropic's invariant: a tool_use part is never sent without its preceding thinking or redacted_thinking block when thinking is enabled.