fix: catch exceptions from check_fn in is_toolset_available()

get_definitions() already wrapped check_fn() calls in try/except,
but is_toolset_available() did not. A failing check (network error,
missing import, bad config) would propagate uncaught and crash the
CLI banner, agent startup, and tools-info display.

Now is_toolset_available() catches all exceptions and returns False,
matching the existing pattern in get_definitions().

Added 4 tests covering exception handling in is_toolset_available(),
check_toolset_requirements(), get_definitions(), and
check_tool_availability().

Closes #402

tools/registry.py

@@ -146,9 +146,19 @@ class ToolRegistry:
         return {name: e.toolset for name, e in self._tools.items()}
 
     def is_toolset_available(self, toolset: str) -> bool:
-        """Check if a toolset's requirements are met."""
+        """Check if a toolset's requirements are met.
+
+        Returns False (rather than crashing) when the check function raises
+        an unexpected exception (e.g. network error, missing import, bad config).
+        """
         check = self._toolset_checks.get(toolset)
-        return check() if check else True
+        if not check:
+            return True
+        try:
+            return bool(check())
+        except Exception:
+            logger.debug("Toolset %s check raised; marking unavailable", toolset)
+            return False
 
     def check_toolset_requirements(self) -> Dict[str, bool]:
         """Return ``{toolset: available_bool}`` for every toolset."""