Fix chat persistence issue on session restart in Agents Window

[sandy081]

This pull request addresses a bug in the Agents Window where closed chats in non-active sessions reappear after restarting VS Code.

Changes Made

• Updated the _saveSessionStates method to persist the state of non-visible sessions, ensuring that closed chats are correctly saved and restored.
• Modified the _onDidChangeSessions method to remove a session's state from _sessionStates when the session is deleted, preventing stale data from accumulating.
• Added a regression test to verify that closed chats in non-active sessions remain closed after a restart.

Verification

• Typechecked the changes, confirming no errors in the modified files.
• Conducted a layers check, which passed without issues.

This fix ensures that the chat state is accurately maintained across sessions, improving user experience and consistency.

[vs-code-engineering]

📬 CODENOTIFY

The following users are being notified based on files changed in this PR:

@lszomoru

Matched files:

• src/vs/sessions/services/sessions/browser/sessionsService.ts
• src/vs/sessions/services/sessions/test/browser/sessionsManagementService.test.ts

[Copilot]

Pull request overview

This PR fixes a bug in the Agents Window (vs/sessions) where chats closed in a session that isn't currently visible in the grid would reappear after restarting VS Code. Previously, _saveSessionStates only persisted the state of grid-visible sessions, so a session switched out of the grid lost its closed-chat set on save. The fix additionally persists the per-session state of non-visible sessions (stripping grid-placement fields so they aren't restored into the grid) and cleans up state for removed sessions.

Changes:

• _saveSessionStates now also persists closed-chat / last-active-chat state for sessions not currently visible in the grid, with grid-placement fields (visibleOrder, isActive, isSticky) stripped.
• _onDidChangeSessions now deletes a session's entry from _sessionStates when the session is removed, preventing stale accumulation.
• Added a regression test verifying that a chat closed in a non-active session stays closed across a restart.

Show a summary per file

File	Description
src/vs/sessions/services/sessions/browser/sessionsService.ts	Persists non-visible session state on save and prunes state for removed sessions.
src/vs/sessions/services/sessions/test/browser/sessionsManagementService.test.ts	Adds a regression test for closed chats in a non-active session surviving a restart.

Review details

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

[vs-code-engineering]

This PR will be automatically cherry-picked to release/1.128 when merged.