fix: keep function calls when reasoning can't be replayed

[TheGreatAxios]

Closes CL-9423

Summary

• When a reasoning signature can't be replayed (the turn was saved with another model, or the signature is tagged for another provider), the request builder now drops only the reasoning item. The function_call and its function_call_output are both kept, as before orphan suppression.
• Orphan suppression dropped the function_call but kept its output, which OpenAI rejects with 400 "No tool call found for function call output", so tool history broke permanently after a model switch.
• The comment on toResponsesItems now describes this.
• New tests cover both cases (model switch, foreign provider tag) and assert the whole input: message, function_call, function_call_output, in order. There were no earlier suppression tests on this branch to update.

Checks

bun run check, bun run build, bun run test:e2e and the Node pack smoke pass locally.