fix(acp): backfill missing text chunks from part updates

[nkosi23]

Issue for this PR

Closes #15613

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

• Some ACP clients/plugins (e.g. JetBrains/GLM plugin) can miss or delay message.part.delta delivery, causing assistant output to appear truncated until a later refresh.
• The agent needs to emit any missing assistant/reasoning chunks when only message.part.updated arrives so ACP clients receive output incrementally.

This PR therefore contains:

• Added textSnapshots: Map<string,string> to packages/opencode/src/acp/agent.ts to track the last-seen text for each part id.
• Handle message.part.updated for text and reasoning parts by computing the missing delta via textDelta(id, next) and emitting an agent_message_chunk or agent_thought_chunk containing only the new text.
• Update message.part.delta handling to keep textSnapshots in sync (store full part text) so delta and updated events don’t cause duplicate emission.

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• I have tested my changes locally
• I have not included unrelated changes in this PR

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found two potentially related PRs that deal with similar concerns about duplicate events and streaming updates:

Potentially Related PRs:

1. PR refactor(opencode): replace reconcile() with path-syntax setStore and delta coalescing for streaming updates #15309: refactor(opencode): replace reconcile() with path-syntax setStore and delta coalescing for streaming updates

   • refactor(opencode): replace reconcile() with path-syntax setStore and delta coalescing for streaming updates #15309
   • Related because it also addresses delta coalescing for streaming updates, which is related to the duplicate emission prevention in PR fix(acp): backfill missing text chunks from part updates #15614

2. PR fix(app): prevent streaming content duplication during event coalescing #14820: fix(app): prevent streaming content duplication during event coalescing

   • fix(app): prevent streaming content duplication during event coalescing #14820
   • Related because it also tackles preventing duplication during event coalescing, similar to the duplicate prevention in this PR

However, neither of these appears to be a direct duplicate of PR #15614 (the current PR), as they address different aspects of the codebase. PR #15614 is specifically focused on backfilling missing text chunks in the ACP agent when message.part.updated arrives without message.part.delta, whereas the other PRs deal with broader streaming and coalescing concerns.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[nkosi23]

@adamdotdevin 🙏