fix(grpc): Read method from handler_call_details for grpcio >= 1.76 compat (#5521)

## Summary

Fixes sync `ServerInterceptor` to work with grpcio >= 1.76 by reading
the method name from `handler_call_details.method` instead of
`context._rpc_event.call_details.method`.

### Problem

grpcio 1.76 introduced **registered method handlers**
(`add_registered_method_handlers`) which resolve RPC methods at the
C-core level. For these registered methods,
`context._rpc_event.call_details.method` returns an empty string,
causing Sentry tracing to silently fail (no transactions, no spans, no
trace IDs in logs).

### Solution

Capture `handler_call_details.method` directly in the closure scope
before defining the `behavior` function. This approach:

1. **Fixes grpcio >= 1.76 compatibility** -
`handler_call_details.method` is always populated by grpcio's
`_find_method_handler`
2. **Avoids race conditions** - No shared instance state; each call
creates its own closure with captured values
3. **Maintains backward compatibility** - Custom `find_name` parameter
still works

### Why Closure Capture Instead of Instance State?

The gRPC sync server uses a `ThreadPoolExecutor` where multiple threads
share the same `ServerInterceptor` instance. Storing state as
`self._handler_call_details` would create a race condition where one
thread could overwrite the value before another thread's `behavior`
closure reads it.

Fixes #5520

Author: yeung108

sentry_sdk/integrations/grpc/server.py

@@ -22,7 +22,7 @@ def __init__(
         self: "ServerInterceptor",
         find_name: "Optional[Callable[[ServicerContext], str]]" = None,
     ) -> None:
-        self._find_method_name = find_name or ServerInterceptor._find_name
+        self._custom_find_name = find_name
 
         super().__init__()
 
@@ -35,9 +35,12 @@ def intercept_service(
         if not handler or not handler.unary_unary:
             return handler
 
+        method_name = handler_call_details.method
+        custom_find_name = self._custom_find_name
+
         def behavior(request: "Message", context: "ServicerContext") -> "Message":
             with sentry_sdk.isolation_scope():
-                name = self._find_method_name(context)
+                name = custom_find_name(context) if custom_find_name else method_name
 
                 if name:
                     metadata = dict(context.invocation_metadata())
@@ -63,7 +66,3 @@ def behavior(request: "Message", context: "ServicerContext") -> "Message":
             request_deserializer=handler.request_deserializer,
             response_serializer=handler.response_serializer,
         )
-
-    @staticmethod
-    def _find_name(context: "ServicerContext") -> str:
-        return context._rpc_event.call_details.method.decode()