streaming: add MessageStreamReducer and MessageStore

[TirmanSidhu]

Summary

• Adds Swift types for new daemon events (message_open / block_open / block_close / message_close).
• Extends existing event types with messageId / blockIndex / seq fields.
• Adds MessageStore (keyed by messageId, blocks indexed by blockIndex).
• Adds MessageStreamReducer with idempotent apply(event:).
• Wires reducer to consume events from EventStreamClient — but UI does NOT render from MessageStore yet (PR 4 swaps the renderer).

Part of plan: streaming-message-architecture.md (PR 3 of 6)

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Replace accumulated tool input instead of appending

When a tool streams multiple tool_input_delta events, this appends each event's content even though the daemon emits the full accumulated JSON (handleToolInputDelta sends event.accumulatedJson). For app/builder tools that produce several input updates, the new store will contain duplicated prefixes such as {"a"{"abc"..., so the PR 4 renderer would show/correlate a corrupted tool input preview. Store the latest msg.content (or append only a true delta) rather than appending the accumulated payload.

Useful? React with 👍 / 👎.

[chatgpt-codex-connector]

Subscribe with the conversation filter

This new reducer subscribes to the global event stream without the broadcastFilter that the legacy chat loop uses, so every live ChatViewModel receives and stores all conversation-scoped message_open/delta events. In contexts with multiple chat view models (pop-outs/background conversations), messageStore will be contaminated with other conversations before the renderer flips to it; pass the same mutable conversation filter into the reducer subscription or otherwise check conversationId before applying.

Useful? React with 👍 / 👎.