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

[nkosi23]

Motivation

• 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.

Description

• 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.

Testing

• Ran typecheck with bun run typecheck which failed in this environment because tsgo is not installed.
• Ran scoped test command bun test src/acp --timeout 30000 which executed successfully but matched no test files.
• Ran full test suite bun test --timeout 30000 which failed in this environment due to missing local dependencies (e.g. zod, remeda, hono) and registry access errors, so CI-style tests could not be completed here.

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Codex Task