Add session restore coalescing and error handling in AgentService

[DonJayamanne]

For #320303

[Copilot]

Pull request overview

This PR improves Agent Host session restoration performance and reliability by preventing duplicate concurrent restore work (especially expensive session catalog refreshes) and by hardening restore error handling. It directly targets the “Switching sessions is too slow” scenario by ensuring repeated restores/subscribes for the same session (including subagent sessions) share the same in-flight restore work.

Changes:

• Coalesce concurrent restoreSession() calls for the same session URI so only one backend restore is performed at a time.
• Coalesce concurrent restores for the same subagent session URI during subscribe/restore flows.
• Add unit tests covering coalescing behavior and ensuring failed restores don’t permanently block retry.

Show a summary per file

File	Description
src/vs/platform/agentHost/node/agentService.ts	Adds in-flight restore tracking for sessions and subagent sessions to avoid duplicated restore work and to ensure in-flight entries are cleared after completion/failure.
src/vs/platform/agentHost/test/node/agentService.test.ts	Adds restore concurrency and retry tests for both normal and subagent session restore paths.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 0

[DonJayamanne]

@roblourens /cc