fix: scope tool interrupt signal per-thread to prevent cross-session leaks (#7930)

The interrupt mechanism in tools/interrupt.py used a process-global
threading.Event. In the gateway, multiple agents run concurrently in
the same process via run_in_executor. When any agent was interrupted
(user sends a follow-up message), the global flag killed ALL agents'
running tools — terminal commands, browser ops, web requests — across
all sessions.

Changes:
- tools/interrupt.py: Replace single threading.Event with a set of
  interrupted thread IDs. set_interrupt() targets a specific thread;
  is_interrupted() checks the current thread. Includes a backward-
  compatible _ThreadAwareEventProxy for legacy _interrupt_event usage.
- run_agent.py: Store execution thread ID at start of run_conversation().
  interrupt() and clear_interrupt() pass it to set_interrupt() so only
  this agent's thread is affected.
- tools/code_execution_tool.py: Use is_interrupted() instead of
  directly checking _interrupt_event.is_set().
- tools/process_registry.py: Same — use is_interrupted().
- tests: Update interrupt tests for per-thread semantics. Add new
  TestPerThreadInterruptIsolation with two tests verifying cross-thread
  isolation.

tools/process_registry.py

@@ -686,7 +686,7 @@ class ProcessRegistry:
             and output snapshot.
         """
         from tools.ansi_strip import strip_ansi
-        from tools.terminal_tool import _interrupt_event
+        from tools.interrupt import is_interrupted as _is_interrupted
 
         try:
             default_timeout = int(os.getenv("TERMINAL_TIMEOUT", "180"))
@@ -723,7 +723,7 @@ class ProcessRegistry:
                     result["timeout_note"] = timeout_note
                 return result
 
-            if _interrupt_event.is_set():
+            if _is_interrupted():
                 result = {
                     "status": "interrupted",
                     "output": strip_ansi(session.output_buffer[-1000:]),