What this PR does

Keep the app-wide stream state Responding while a local slash command is prepared. A separate command-facing idle ref lets idle-only commands run only when dispatch began from an idle session and no model stream is active. If asynchronous message logging is cancelled, the aborted submission returns before invoking the slash handler; the command-facing flag resets after success and exceptions.

Why it's needed

Fixes #12224. submitQuery marks a user submission as responding before prepareQueryForLlm invokes a UI-only slash handler, so /cd sees an active response and refuses to change directory. Making the app-wide state idle would also let cancellation, telemetry, and other activity-aware consumers treat preparation as finished. A dedicated command-facing ref preserves those app-wide behaviors while allowing the command-specific guard to admit the command.

Reviewer Test Plan

How to verify

• Start an interactive session from idle, run /cd <directory>, and confirm the command changes the session directory.
• Hold logger.logMessage during /cd, press Esc, and confirm the slash handler and model stream are not invoked.
• Hold a local slash dispatch, start a detached ToolResult continuation, and confirm the command-facing idle signal becomes false as soon as the model stream starts, without forcing a rerender.
• Start a main response, submit /btw, then finish the main stream while the side dispatch remains held; confirm the command-facing idle signal stays false.
• Cancel slash dispatch A while its logger is held, start B, then release A and confirm B keeps the command-facing idle signal until B settles.
• In YOLO mode, update history twice during a local slash command and confirm one conversation_finished event is emitted after settlement.

Evidence (Before & After)

Before: the Windows reproduction attached to #12224 shows /cd refused with “Cannot change directory while a response or tool call is in progress.” After: use-llm-stream.test.tsx and AppContainer.test.tsx cover synchronous stream-state reads, per-dispatch cleanup, cancellation, detached continuations, /btw, ordinary idle behavior, app-wide ref isolation, throw cleanup, and YOLO telemetry.

Tested on

Environment (optional)

Windows 11, Node.js v24.15.0, Qwen Code worktree. The global qwen executable is not installed, so no live CLI smoke was run; the delayed-logger hook tests provide the deterministic reproduction. GitHub CI and review status are tracked against the latest PR head.

Risk & Scope

• Main risk or tradeoff: slash-command guards see idle only during an idle-origin local dispatch with no active model stream; the app-wide activity state remains Responding.
• Not validated / out of scope: live cross-platform TUI smoke on macOS or Linux.
• Breaking changes / migration notes: none.

Linked Issues

Fixes #12224