Stream text/reasoning deltas to ACP and deduplicate using snapshots

[nkosi23]

Motivation

• Ensure incremental text and reasoning updates are forwarded to ACP when message.part.updated events arrive without explicit delta events.
• Prevent duplicated chunks when a message.part.delta is followed by a message.part.updated that includes the same content.

Description

• Add a textSnapshots map to track the last full text for each part via textSnapshots.
• Add a textDelta helper that computes the incremental delta from the previous snapshot and updates the snapshot.
• Send incremental agent message chunks for text and reasoning parts in the message.part.updated flow using textDelta.
• Update message.part.delta handling to set the snapshot (textSnapshots.set) when full text is available to avoid replaying the same content.
• Add unit tests in packages/opencode/test/acp/event-subscription.test.ts to verify that text chunks are emitted for message.part.updated and that duplicate chunks are not produced when a delta is followed by an updated event.

Testing

• Ran the ACP event subscription tests in packages/opencode/test/acp/event-subscription.test.ts via yarn test for the package and the new tests passed.
• The two added tests (emits text chunks from message.part.updated when delta events are absent and does not duplicate chunks when message.part.delta is followed by message.part.updated) succeeded as part of the suite.

---

Codex Task