agentHost: Address session catalog review feedback

[Sandeep Somavarapu (sandy081)]

Follow-up to #331679.

Addresses the post-merge review feedback:

• Reapply current workspace filtering to the retained session-list projection before a failed backend refresh can leave stale entries visible.
• Replace the ineffective epoch-race test with a registry snapshot race that proves an already-started listing restarts after mutation.
• Document scheduled backoff versus trigger-driven retries accurately.

The retained projection intentionally only narrows while offline; newly eligible sessions arrive through the next successful listing or session-added notification.

Validation

• npm run typecheck-client
• npm run compile
• npm run hygiene
• 660 affected tests passed
• git diff --check

[Copilot]

Pull request overview

Hardens Agent Host session catalog behavior during workspace changes and concurrent registry mutations.

Changes:

• Re-filters retained sessions before failed refreshes can expose stale entries.
• Replaces the epoch race test with a registry snapshot race test.
• Clarifies retry and backoff documentation.

Show a summary per file

File	Description
agentHostSessionListStore.ts	Narrows cached sessions on workspace changes.
agentHostChatContribution.test.ts	Tests filtering when refresh fails.
agentService.test.ts	Verifies listing restarts after registry mutation.
AGENTS.md	Documents client-specific retry behavior.

Review details

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• Files reviewed: 4/4 changed files
• Comments generated: 0
• Review effort level: Balanced