fix(#2791): await session.abort() in all subagent completion/cancel paths

Fire-and-forget session.abort() calls during subagent completion left
dangling promises that raced with parent session teardown. In Bun on
WSL2/Linux, this triggered a StringImplShape assertion (SIGABRT) as
WebKit GC collected string data still referenced by the inflight request.

Fix: await session.abort() in all four completion/error paths:
- startTask promptAsync error handler (launch path)
- resume promptAsync error handler (resume path)
- cancelTask (explicit cancel path)
- tryCompleteTask (normal completion path)

Also marks the two .catch() error callbacks as async so the await is
valid.

Test: update session.deleted cascade test to flush two microtask rounds
since cancelTask now awaits abort before cleanupPendingByParent.

src/features/background-agent/manager.test.ts

@@ -3728,6 +3728,9 @@ describe("BackgroundManager.handleEvent - session.deleted cascade", () => {
       properties: { info: { id: parentSessionID } },
     })
 
+    // Flush twice: cancelTask now awaits session.abort() before cleanupPendingByParent,
+    // so we need additional microtask ticks to let the cascade complete fully.
+    await flushBackgroundNotifications()
     await flushBackgroundNotifications()
 
     // then