[cherry-pick] agentHost: Fix sessions stuck in progress after subagent completion

[vs-code-engineering]

Cherry-pick of #334781 from main.

Summary

Fix completed Copilot Agent Host sessions remaining In Progress in the Agents window after a subagent follow-up finishes.

The observed SDK sequence supplies the subagent ID in hook.start.data.input.agentId for a subagentStop hook, but omits it from the hook.end envelope. Our handler only completed subagents from agentStop events with an envelope agent ID, leaving the follow-up turn active. #333999 made that stale child status visible on the parent session row.

Fix

• Remember the subagent ID by hookInvocationId until the matching stop hook ends.
• Reuse the existing subagent completion path and remove the correlation immediately.
• Keep the sessions-list behavior unchanged; no polling, timeouts, or forced UI status.
• Cover follow-up completion without an envelope agent ID, overlapping hooks, and duplicate completions that must not close a later turn.

Validation

• Both new regression tests failed before the fix.
• All 430 tests in copilotAgentSession.test.ts passed afterward.
• npm run transpile-client
• Targeted ESLint and staged hygiene (npm run precommit)
• git diff --check

Release cherry-pick requested via ~release-cherry-pick.

[Copilot]

🟢 Approval recommended

The focused fix is correctly guarded against duplicate completion and has comprehensive regression coverage.

Pull request overview

Fixes Agent Host sessions remaining in progress after subagent follow-up completion.

Changes:

• Correlates subagent stop-hook starts and ends by invocation ID.
• Adds regression coverage for missing IDs, overlapping hooks, and duplicate completions.

File summaries

File	Description
copilotAgentSession.ts	Completes correlated subagent stop hooks.
copilotAgentSession.test.ts	Tests completion and correlation edge cases.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Balanced

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