feat: Implement changeset state caching and management in AgentHost

[DonJayamanne]

Problem today

• we have 1000 sessions
• listSessions will build the state for the session
• We load the changeset, but only count number of changes +added -deleted
• However we also load and store the state of changesets for each session in memory
• However since none of the sessions have been opened (not subscribed), this changeset information for all 1000 sessions is a waste of resources, ideally we should load this into state as and when required (when a session is opened)

Fix

• we have 1000 sessions
• listSessions will build the state for the session
• We load the changeset, but only count number of changes +added -deleted
• No more loading changesets into memory unless its requested

[Copilot]

Pull request overview

Refactors expanded changeset state retention out of AgentHostStateManager into a dedicated AgentHostChangesetStateCache with a soft LRU policy, and reshapes persisted-changeset handling so the session list overlay parses metadata without forcing live state to be seeded for every row.

Changes:

• Adds AgentHostChangesetStateCache (silent LRU, soft limit 500) and routes all changeset state access in AgentHostStateManager through it.
• AgentService supplies subscriber/compute-aware canEvict and calls onChangesetLivenessChanged on unsubscribe; AgentHostChangesetService tracks _activeStaticComputes and pings liveness on completion/failure.
• Splits restorePersistedStaticChangesets into parsePersistedStaticChangesets + applyPersistedStaticChangesets; ChangesetSessionCoordinator.decorateListEntry now only parses (does not seed) for the list overlay.

Show a summary per file

File	Description
src/vs/platform/agentHost/node/agentHostChangesetStateCache.ts	New LRU-style soft-limit cache with canEvict hook and silent eviction.
src/vs/platform/agentHost/node/agentHostStateManager.ts	Delegates changeset state storage to the cache; adds onChangesetLivenessChanged and constructor options.
src/vs/platform/agentHost/node/agentHostChangesetService.ts	Adds parse/apply split, isStaticChangesetComputeActive, and active-compute tracking around _doComputeStaticChangeset.
src/vs/platform/agentHost/node/agentHostChangesetCoordinator.ts	List overlay now calls parsePersistedStaticChangesets instead of restore (no state seeding).
src/vs/platform/agentHost/node/agentService.ts	Wires canEvict to subscriber/subagent/active-compute checks; notifies liveness on unsubscribe.
src/vs/platform/agentHost/test/node/agentHostChangesetService.test.ts	Adds parse/apply and LRU eviction test coverage.
src/vs/platform/agentHost/test/node/agentSideEffects.test.ts	Updates FakeChangesetService with new no-op methods.

Copilot's findings

• Files reviewed: 7/7 changed files
• Comments generated: 1