feat(core): implement mid-turn queue drain for agent execution (#2854)

* feat(core): implement mid-turn queue drain for agent execution

Inject queued user messages between tool execution steps within a single
turn, so the model sees them immediately instead of waiting for the
entire round to complete.

- Add `dequeueAll()` to AsyncMessageQueue
- Add `midTurnDrain` callback to ReasoningLoopOptions
- Drain queue after processFunctionCalls, inject as text parts
- AgentComposer always enqueues directly (no local buffering)
- Add QUEUE_MESSAGES_CONSUMED event for UI sync

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* feat(cli): add mid-turn queue drain to main session

Extend mid-turn queue drain to the main session's tool execution path
(useGeminiStream). Previously only agent tabs had this feature.

- Add midTurnDrainRef parameter to useGeminiStream
- Inject queued messages in handleCompletedTools before submitQuery
- Bridge useMessageQueue to drain ref in AppContainer via ref pattern

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: address Copilot review feedback on mid-turn drain

- Guard midTurnDrain with abort check to prevent message loss on cancel
- Synchronously clear messageQueueRef to prevent duplicate drains
- Only clear pending display on IDLE status, not all status changes

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* refactor: scope mid-turn drain to main session only

Revert subagent-path changes (AgentCore, AgentInteractive,
AgentComposer, AsyncMessageQueue, agent-events) to keep the PR
focused on the main session, which is easier to test and validate.

Subagent mid-turn drain can be added in a follow-up PR.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: address Copilot review on main session mid-turn drain

- Move synchronous queue ref into useMessageQueue itself, expose
  drainQueue() for atomic drain (fixes race between addMessage and drain)
- Record drained messages as USER history items so the transcript
  stays complete
- Simplify AppContainer bridge to just midTurnDrainRef.current = drainQueue

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: guard mid-turn drain against cancelled turns

- Skip drain when turnCancelledRef or abortController signal is set,
  so queued messages stay for the next turn instead of being lost
- Restore ref-based queue bridge (drainQueue removed from useMessageQueue)
- Keep synchronous ref clear to prevent duplicate drains

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/cli/src/ui/hooks/useGeminiStream.ts

@@ -173,6 +173,7 @@ export const useGeminiStream = (
   setShellInputFocused: (value: boolean) => void,
   terminalWidth: number,
   terminalHeight: number,
+  midTurnDrainRef?: React.RefObject<(() => string[]) | null>,
 ) => {
   const [initError, setInitError] = useState<string | null>(null);
   const abortControllerRef = useRef<AbortController | null>(null);
@@ -1572,6 +1573,23 @@ export const useGeminiStream = (
         return;
       }
 
+      // Mid-turn queue drain: inject queued user messages alongside tool
+      // results so the model sees them in the next API call.
+      // Skip if the turn was cancelled — messages stay in queue for next turn.
+      const drained =
+        turnCancelledRef.current || abortControllerRef.current?.signal.aborted
+          ? []
+          : (midTurnDrainRef?.current?.() ?? []);
+      if (drained.length > 0) {
+        for (const msg of drained) {
+          responsesToSend.push({
+            text: `\n[User message received during tool execution]: ${msg}`,
+          });
+          // Record in UI history so the transcript stays complete.
+          addItem({ type: MessageType.USER, text: msg }, Date.now());
+        }
+      }
+
       submitQuery(responsesToSend, SendMessageType.ToolResult, prompt_ids[0]);
     },
     [
@@ -1582,6 +1600,8 @@ export const useGeminiStream = (
       performMemoryRefresh,
       modelSwitchedFromQuotaError,
       config,
+      midTurnDrainRef,
+      addItem,
     ],
   );