Fix hang in AH evals

[roblourens]

Fix microsoft/vscode-copilot-evaluation#5242

[Copilot]

Pull request overview

This PR fixes a hang in Agent Host (AH) evals (microsoft/vscode-copilot-evaluation#5242). The root cause: two hydration-await sites in AgentHostSessionHandler waited on a subscription using an onDidChange-only Promise. When a subscribe fails, BaseAgentSubscription.setError fires onDidError but not onDidChange, so those awaits never woke and the turn parked for the full harness timeout. The fix routes both sites through the pre-existing _whenSubscriptionHydrated helper (already used at 4 other call sites), which settles on snapshot, error, or cancellation.

Changes:

• Replaced the inline onDidChange-only await in _readEagerlyCreatedSessionState (~line 1258) with _whenSubscriptionHydrated.
• Replaced the inline onDidChange-only await in the create-and-subscribe path (~line 3543) with _whenSubscriptionHydrated.
• Added a regression test plus a MockAgentHostService "pending→error" subscription helper that reproduces the setError semantics (fires onDidError, never onDidChange).

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostSessionHandler.ts	Routes two remaining inline hydration awaits through _whenSubscriptionHydrated so a failed subscribe (onDidError) no longer hangs the turn.
src/vs/workbench/contrib/chat/test/browser/agentSessions/agentHostChatContribution.test.ts	Adds a pending→error subscription mock and a regression test verifying the handler recovers instead of hanging (repro for #5242).

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Medium