Preserve sandbox history when live hydration fails - #340815

Summary

Follow-up to #340354, addressing the live-hydration review finding. No separate tracking issue was supplied.

AgentHostSessionHandler can fulfill its content request with an error transcript or an empty root-session fallback after a subscription failure. The sandbox facade previously treated either result as a successful live snapshot, replacing recorded messages, invalidating their cache, cancelling the recorded refresh, and disabling its recovery paths.

• Expose an optional, explicit initial-history failure outcome rather than inferring failure from empty history or historical error responses. Other consumers keep their existing error rendering.
• Reject failed live promotion while retaining the recorded transcript, cache, ongoing history refresh, and existing live Retry action. Release the rejected session and its subscriptions, including failures before a chat URI was resolved.
• Preserve genuinely new sessions, successfully hydrated empty chats, and conversations containing failed historical turns. A missing root session can still initialize lazily, but only after the already-running recorded read successfully confirms there is no conversation to preserve.
• A truncated replay that omits the requested root reports incomplete history, not an empty conversation. Partial transcripts containing the requested root remain readable with their existing warning.

No new setting, protocol field, storage key, telemetry event, or banner is introduced. Recorded-history recovery still does not reconnect or submit input.

Validation

Truncated-root follow-up (d9b11a29af2), using Node 24.18.0:

• npm run build-fast -- --client-only — passed.
• node build/hygiene.ts src/vs/workbench/contrib/chat/browser/remoteAgentHost/cloudSandboxReadOnlySessionHandler.ts src/vs/workbench/contrib/chat/test/browser/agentSessions/agentHostChatContribution.test.ts — passed.
• npm run eslint -- src/vs/workbench/contrib/chat/browser/remoteAgentHost/cloudSandboxReadOnlySessionHandler.ts src/vs/workbench/contrib/chat/test/browser/agentSessions/agentHostChatContribution.test.ts — passed.
• git diff --check — passed.
• The following selection passed 86 tests, including both new missing-root truncation regressions and the existing readable-partial-history case. Both new cases failed before the one-line validation fix.

xvfb-run -a ./scripts/test.sh --no-sandbox --disable-dev-shm-usage --reporter dot \
  --run src/vs/workbench/contrib/chat/test/browser/agentSessions/agentHostChatContribution.test.ts \
  --run src/vs/workbench/contrib/chat/test/browser/remoteAgentHost/cloudSandboxSessionHandler.test.ts \
  --run src/vs/workbench/contrib/chat/test/browser/remoteAgentHost/cloudSandboxHistoryCache.test.ts \
  --grep 'history loading cloud sandbox promotion|CloudSandboxSessionHandler|CloudSandboxHistoryCache'

Initial fix (e5ac639c4b8), using Node 24.18.0:

• npm run install-fast — passed after the new main branch's SDK/runtime version check identified stale dependencies. No dependency manifests changed.
• npm run build-fast -- --client-only — passed.
• npm run typecheck-client — passed, including the final cancellation/subscription-release assertions.
• node build/hygiene.ts src/vs/workbench/contrib/chat/common/chatSessionsService.ts src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostSessionHandler.ts src/vs/workbench/contrib/chat/browser/remoteAgentHost/cloudSandboxSessionHandler.ts src/vs/workbench/contrib/chat/test/browser/agentSessions/agentHostChatContribution.test.ts — passed. After adding cancellation/subscription-release coverage, node build/hygiene.ts src/vs/workbench/contrib/chat/test/browser/agentSessions/agentHostChatContribution.test.ts also passed.
• npm run eslint -- src/vs/workbench/contrib/chat/common/chatSessionsService.ts src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostSessionHandler.ts src/vs/workbench/contrib/chat/browser/remoteAgentHost/cloudSandboxSessionHandler.ts src/vs/workbench/contrib/chat/test/browser/agentSessions/agentHostChatContribution.test.ts — passed, all four changed files.
• git diff --check — passed.

The following focused selection passed 130 tests, including 12 real-provider sandbox promotion cases:

xvfb-run -a ./scripts/test.sh --no-sandbox --disable-dev-shm-usage --reporter dot \
  --run src/vs/workbench/contrib/chat/test/browser/agentSessions/agentHostChatContribution.test.ts \
  --run src/vs/workbench/contrib/chat/test/browser/remoteAgentHost/cloudSandboxSessionHandler.test.ts \
  --run src/vs/workbench/contrib/chat/test/browser/remoteAgentHost/cloudSandboxHistoryCache.test.ts \
  --grep 'history loading|history with tool events|disposal|does not render a load failure|still renders a load failure|CloudSandboxSessionHandler|CloudSandboxHistoryCache'

The real-provider regression cases reproduced eight failures on the unfixed implementation, including cache invalidation, transcript replacement, cancelled recorded reads, and lost retry. One intermediate Electron launch crashed during module loading (exit code 5), before executing tests; the final identical selection completed successfully.

Live cloud end-to-end flows were not exercised.

How to Test

1. Open a sandbox with cached or recorded messages and make its live session/default-chat/peer-chat subscription fail. Confirm messages and the cache remain intact, the recorded API refresh continues, and the existing live Retry action remains available.
2. Let the recorded refresh succeed, then recover the live subscription and invoke Retry. Confirm the same chat promotes successfully without dropping the draft.
3. Repeat with a root-session NotFound response. Existing recorded messages must remain readable; a successful empty recorded result must still allow lazy session initialization. An unavailable API or a truncated replay missing the requested root must not be mistaken for an empty conversation, including when the replay contains a different session.
4. Check a new session, an empty but successfully hydrated chat, and a transcript containing a failed turn. All must still load normally. Close a chat while a missing-root result is waiting for recorded history and confirm its pending work is cancelled and released.