[Raing5Days]

Fixes #3729.

Problem

When switching sessions/workspaces — especially across a local <-> remote workspace switch — passive host dialog queue observers (observeHostQueue / useHostQueueTimeline) keep polling manage_dialog_queue for a session that has not been restored on the execution host. The backend (DialogScheduler::execute_queue_request) rejects every read with NotFound: "Session is not loaded on the execution host", and the transport adapter logs each poll as ERROR (~every 10s; the issue trace shows 7 failures in 76 seconds).

Two secondary symptoms in the same trace:

• Every workspace switch emits workspace:loading immediately followed by workspace:error (with a null error) in the same tick, so listeners observe an instant-failure pair even for clean switches.
• Polls of get_external_source_* against a host without the external compatibility service log ERROR on every poll even though callers degrade by design.

Root cause

1. execute_queue_request bails out with PortErrorKind::NotFound before looking at the action whenever session_manager.get_session(session) is None. But the scheduler keeps its own queue receipts independent of session restoration, so read-only requests could be answered correctly.
2. WorkspaceManager.setLoading / setError emit unconditionally; setLoading(true) followed by setError(null) emits workspace:error with an unchanged null error.
3. The transport adapter's expected-error classifier only knew about get_config.

Changes

• DialogScheduler::execute_queue_request (host_message_queue.rs): read-only actions (List, Get) now answer from the scheduler-owned receipts even when the session is not loaded on this execution host; mutations (Submit, Cancel, Promote) still refuse user work for an unloaded session so nothing can silently disappear.
• WorkspaceManager.setLoading / setError: state-quiet — skip the update and event when the value did not change.
• TauriTransportAdapter.isExpectedTauriRequestError: classify a structured host_unavailable answer (ExternalSourceOperationError contract, direct object or serialized inside message/error/details) from get_external_source_* polls as expected (debug-level). ApiClient reuses the same classifier for the command-level log path.

Testing

• New regression test host_queue_reads_degrade_for_a_session_not_loaded_on_the_host submits a message, evicts the loaded session, and asserts List/Get still return the queued receipt with a stable epoch while Submit/Promote fail with NotFound. Full host_queue_ filter: 20 passed.
• New workspaceManager tests: a clean scanWorkspaceInfo emits workspace:loading but no workspace:error; a real failure still reports workspace:error. 35 passed.
• New tauri-adapter tests: structured host_unavailable on get_external_source_* is expected; plain prose failures and mutation commands are not. 10 passed. HostPendingQueuePanel tests stay green (4 passed; 49 total across the three files).
• pnpm run type-check (web-ui) passes; pnpm run fmt:rs applied.

AI-assisted.