fix(mcp): budget discovery-thread GIL so agent build fits the 30s wait

Background MCP discovery does bursty CPU work (SDK/pydantic imports, JSON-RPC
schema parsing, tool registration) that at the default 5ms switch interval rides
the GIL convoy effect and starves the concurrent agent-build thread and the main
event loop for tens of seconds after a serve restart — _wait_agent(30s) times
out with 'agent initialization timed out' (error 5032).

Lower the interpreter switch interval while discovery runs (restored afterwards)
so its CPU bursts are sliced finely enough that waiters stay responsive.

Fixes #60371

hermes_cli/mcp_startup.py

@@ -2,10 +2,11 @@
 
 from __future__ import annotations
 
+import sys
 import threading
-from contextlib import nullcontext
+from contextlib import contextmanager, nullcontext
 from contextvars import copy_context
-from typing import Dict, Optional, Set
+from typing import Dict, Iterator, Optional, Set
 
 from hermes_constants import hermes_home_key
 
@@ -158,6 +159,27 @@ def _resolve_discovery_timeout(explicit: "float | None", *, single_query: bool =
         return default
 
 
+# GIL budget for discovery: the discovery thread does bursty CPU work (mcp/pydantic imports,
+# JSON-RPC schema parsing, tool registration) that, at the default 5 ms switch interval, rides
+# the GIL convoy effect and can starve concurrent threads — the agent-build thread and the main
+# event loop — for tens of seconds (#60371: "agent initialization timed out" after a serve
+# restart, _wait_agent(30s) error 5032). While discovery runs we drop the switch interval so its
+# CPU bursts are sliced finely enough that waiters stay responsive, then restore it.
+_DISCOVERY_SWITCH_INTERVAL_S = 0.0005
+
+
+@contextmanager
+def _discovery_gil_budget() -> Iterator[None]:
+    """Temporarily lower the interpreter switch interval so discovery's CPU work can't
+    monopolize the GIL against concurrent threads (see _DISCOVERY_SWITCH_INTERVAL_S)."""
+    prev = sys.getswitchinterval()
+    sys.setswitchinterval(_DISCOVERY_SWITCH_INTERVAL_S)
+    try:
+        yield
+    finally:
+        sys.setswitchinterval(prev)
+
+
 def _discover_mcp_tools_without_interactive_oauth() -> None:
     """Run MCP discovery without letting OAuth read from the user's stdin."""
     try:
@@ -165,7 +187,7 @@ def _discover_mcp_tools_without_interactive_oauth() -> None:
     except Exception:
         suppress_interactive_oauth = nullcontext
 
-    with suppress_interactive_oauth():
+    with _discovery_gil_budget(), suppress_interactive_oauth():
         from tools.mcp_tool_discovery import discover_mcp_tools
 
         # Only pass the kwarg when a filter is set: many tests (and any