fix(sessions): snapshot send target so a project switch cannot reroute a pending send (#2871)

Snapshot the new-session draft (and keep the existing-session target captured) at
submit time, then use that snapshot for draft materialization and routing instead
of re-reading live selection state after async preparation.

Fixes #2222
Fixes #2315

packages/ui/src/sync/session-ui-store.ts

@@ -231,6 +231,8 @@ type SendMessageOptions = {
   target?: CapturedSendTarget
   sessionId?: string
   directory?: string
+  /** Immutable copy of the new-session draft at submit time; used instead of the live draft. */
+  draftSnapshot?: NewSessionDraftState
   delivery?: 'steer'
 }
 
@@ -609,9 +611,9 @@ export async function materializeOpenDraftSession(selection: {
   modelID: string
   agent?: string
   variant?: string
-}): Promise<MaterializedDraftSession | null> {
+}, draftOverride?: NewSessionDraftState): Promise<MaterializedDraftSession | null> {
   const store = useSessionUIStore.getState()
-  const draft = store.newSessionDraft
+  const draft = draftOverride ?? store.newSessionDraft
   if (!draft?.open) return null
   const draftPermissionAutoAcceptEnabled = draft.permissionAutoAcceptEnabled === true
 
@@ -1224,7 +1226,7 @@ export const useSessionUIStore = create<SessionUIState>()((set, get) => ({
       set({ pendingChangesBarDismissed: map });
     }
 
-    const draft = get().newSessionDraft
+    const draft = options?.draftSnapshot ?? get().newSessionDraft
     const trimmedAgent = typeof agent === "string" && agent.trim().length > 0 ? agent.trim() : undefined
 
     const goalArm = inputMode !== "shell" && content.trim().length > 0
@@ -1282,7 +1284,7 @@ export const useSessionUIStore = create<SessionUIState>()((set, get) => ({
         modelID,
         agent: trimmedAgent,
         variant,
-      })
+      }, options?.draftSnapshot)
       if (!createdDraftSession) throw new Error("Failed to create session")
 
       const mergedAdditionalParts = createdDraftSession.syntheticParts?.length