test: add regression test for parent_session_id task result routing

[randomm]

Task Description

PR #179 (commit fbeb585ea) fixed the async task result delivery bug by adding parent_session_id to TaskMetadata and routing results via parent session ID. However, no regression test was added for this specific routing logic.

If someone swaps parent_session_id back to session_id in getAndClearCompletedTasks() or hasUndeliveredCompletedTasks(), no test would catch it.

What to Test

File: packages/opencode/test/session/async-tasks.test.ts (new file, or add to existing test)

1. Result retrieval by parent session ID: Call trackBackgroundTask() with a child session ID as 3rd arg and parent_session_id set to a different parent ID in metadata. Then call getAndClearCompletedTasks(parentID) — should find the result. Call with getAndClearCompletedTasks(childID) — should NOT find it.

2. hasUndeliveredCompletedTasks routing: Same setup — hasUndeliveredCompletedTasks(parentID) should return true, hasUndeliveredCompletedTasks(childID) should return false.

3. Event bus parentSessionID: When task completes, the BackgroundTaskEvent.Completed event should have parentSessionID matching the parent_session_id from metadata, NOT session_id.

4. Cleanup uses child session_id: cleanupSessionMaps(childID) should clean up the task. cleanupSessionMaps(parentID) should NOT clean up the child's task.

Key Files

• packages/opencode/src/session/async-tasks.ts — functions under test
• packages/opencode/test/tool/check_task.test.ts — existing task tests for reference

Quality Gates (Non-Negotiable)

• TDD: Tests written to fail if routing reverts to session_id
• Coverage: Cover all 4 scenarios above
• Linting: All code passes project linting rules
• Local Verification: bun test passes

Acceptance Criteria

• New test file or test block that specifically validates parent_session_id routing
• Tests fail if parent_session_id is swapped back to session_id in retrieval functions
• All existing tests continue to pass