feat: enhance session changeset handling with deferred refresh logic

[DonJayamanne]

No description provided.

[Copilot]

Pull request overview

Adds a deferred-refresh path for session-wide ("static") changesets that mirrors the existing uncommitted-refresh deferral: when a subscriber arrives before the session's working directory is known, the refresh is queued and drained on session materialization/restore, and dropped if the last subscriber leaves first.

Changes:

• New _pendingSessionRefreshes set with _triggerSessionRefresh helper, drained alongside uncommitted refreshes and cleared on disposal / last-subscriber.
• Both onFirstSubscriber Session-kind and Uncommitted fall-through paths now route through the new helper instead of calling refreshSessionChangeset directly.
• Added two unit tests covering deferred refresh on materialization and drop-on-unsubscribe.

Show a summary per file

File	Description
src/vs/platform/agentHost/node/agentHostChangesetCoordinator.ts	Adds deferred session-changeset refresh queue, helper, drain/cleanup; updates docstrings (one got corrupted)
src/vs/platform/agentHost/test/node/agentHostChangesetCoordinator.test.ts	Adds tests for defer-until-materialized and drop-on-last-subscriber behaviors

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 1