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

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PR Summary

Medium Risk
Breaking public API (messages → chatHistory) and stricter protocol parsing (response_id required) affect all transcript UIs; core reconciliation logic changed but is covered by new unit tests.

Overview
Replaces the public messages array with chatHistory — a single ordered list of ChatHistoryItem entries (messages plus ConversationToolCall records). Conversation, ConversationClient, docs, and the chat widget now observe $chatHistory and derive transcript bubbles via compactMap(\.message).

Incoming agent events (streaming parts, full responses, corrections) are merged through a new ChatHistoryReconciler, keyed by server response_id so multiple agent utterances in one turn (e.g. before/after a tool) no longer collide on event_id. EventParser and public event types now require response_id for those agent payloads. Message gains isFinal and stable ids (agent messages use response_id); tentative user transcripts update history instead of being ignored.

The widget shows a typing cursor on non-final bubbles; ChatWidgetController.messages() becomes chatHistory().

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