fix: drop thinking messages without signature

[uinstinct]

Description

Prevent thinking messages without signatures to be sent to the LLM.

resolves CON-4363

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• [] I've read the contributing guide
• [] The relevant docs, if any, have been updated or created
• [] The relevant tests, if any, have been updated or created

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after.mp4

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Summary by cubic

Prevent sending “thinking” messages without a signature to the LLM to avoid incomplete thoughts being processed. Only include messages with a signature or redactedThinking (addresses Linear CON-4363).

[cubic-dev-ai]

No issues found across 1 file

[RomneyDa]

I think only anthropic API has this limitation

[uinstinct]

I think only anthropic API has this limitation

I see. Do we want to include the thinking portion when it is incomplete?

[RomneyDa]

I think if you include the thinking portion without a signature it errors, so we can fully ignore thinking messages that don't have signatures at the anthropic llm class/openai adapter level

[RomneyDa]

@uinstinct I did this as a hotfix for haiku 4.5 here
#8329