fix: prevent background agent race condition in session prompt (#2932)

Added await for session ready state before sending prompt in
background-agent/manager.ts. Also improved image resizer error handling.

132 tests pass, tsc clean.

Closes #2932

src/tools/delegate-task/background-task.test.ts

@@ -345,6 +345,54 @@ describeFn("executeBackgroundTask output/session metadata compatibility", () =>
     expectFn(result).toContain("interrupt")
   })
 
+  testFn("reports failure when manager marks task as error during session startup", async () => {
+    //#given - session created but startTask throws before prompt is sent
+    const metadataCalls: any[] = []
+    let reads = 0
+    const manager = {
+      launch: async () => ({
+        id: "bg_crash_before_prompt",
+        sessionID: undefined,
+        description: "Crash before prompt",
+        agent: "explore",
+        status: "pending",
+      }),
+      getTask: () => {
+        reads += 1
+        if (reads >= 2) {
+          return { sessionID: "ses_orphan", status: "error", error: "crash between session creation and prompt send" }
+        }
+        return { sessionID: undefined, status: "pending" }
+      },
+    }
+
+    //#when
+    const result = await executeBackgroundTask(
+      {
+        description: "Crash before prompt",
+        prompt: "check",
+        run_in_background: true,
+        load_skills: [],
+      },
+      {
+        sessionID: "ses_parent",
+        callID: "call_crash",
+        metadata: async (value: any) => metadataCalls.push(value),
+        abort: new AbortController().signal,
+      },
+      { manager },
+      { sessionID: "ses_parent", messageID: "msg_crash" },
+      "explore",
+      undefined,
+      undefined,
+      undefined,
+    )
+
+    //#then - polling loop should detect terminal status and report failure
+    expectFn(result).toContain("Task failed to start")
+    expectFn(result).toContain("error")
+  })
+
   testFn("keeps sibling background launch alive when two tasks start concurrently", async () => {
     //#given - one aborted parent call should not interrupt a sibling launch from the same parent session
     const firstAbortController = new AbortController()