Improve message reconciliation

[renal128]

Summary

Turns out that event_id is not a unique ID - there can be multiple agent responses with the same event_id (e.g. if theres agent utterance, tool call and another agent utterance in the same turn). So we didn't have a good way to match streamed events. Recently we added responseId in the backend as unique identifiers of agent utterances, so we can match them properly, which is the focus of this PR:

• Reconcile agent streams, full responses, and corrections by response_id (one turn can produce several agent messages).
• Replace messages with chatHistory: a mixed, stable list of messages and tool calls, that can be extended with other types of entries in the future.

Test plan

• unit tests
• manual tests

[cursor]

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