fix: route /bg spinner through TUI widget to prevent status bar collision (#3643)

Background agent's KawaiiSpinner wrote \r-based animation and stop()
messages through StdoutProxy, colliding with prompt_toolkit's status bar.

Two fixes:
- display.py: use isinstance(out, StdoutProxy) instead of fragile
  hasattr+name check for detecting prompt_toolkit's stdout wrapper
- cli.py: silence bg agent's raw spinner (_print_fn=no-op) and route
  thinking updates through the TUI widget only when no foreground
  agent is active; clear spinner text in finally block with same guard

Closes #2718

Co-authored-by: kshitijk4poor <kshitijk4poor@users.noreply.github.com>

agent/display.py

@@ -284,11 +284,11 @@ class KawaiiSpinner:
         The CLI already drives a TUI widget (_spinner_text) for spinner display,
         so KawaiiSpinner's \\r-based animation is redundant under StdoutProxy.
         """
-        out = self._out
-        # StdoutProxy has a 'raw' attribute (bool) that plain file objects lack.
-        if hasattr(out, 'raw') and type(out).__name__ == 'StdoutProxy':
-            return True
-        return False
+        try:
+            from prompt_toolkit.patch_stdout import StdoutProxy
+            return isinstance(self._out, StdoutProxy)
+        except ImportError:
+            return False
 
     def _animate(self):
         # When stdout is not a real terminal (e.g. Docker, systemd, pipe),