test: JSON-RPC-specific dispatcher tests + coverage to 100%

Covers behaviors with no DirectDispatcher analog: out-of-order response
correlation, INTERNAL_ERROR over the wire, peer-cancel in interrupt and signal
modes, CONNECTION_CLOSED on stream EOF mid-await, late-response drop,
raise_handler_exceptions propagation, ServerMessageMetadata tagging on
ctx.send_request, null-id JSONRPCError drop, ValidationError->INVALID_PARAMS,
contextvar propagation via _spawn, and the defensive Broken/Closed/WouldBlock
catches.

Two small src tweaks for coverage:
- _cancel_outbound: combine the two except arms into one tuple
- _dispatch: pragma no-branch on the final case (match is exhaustive over
  JSONRPCMessage; the no-match arc is unreachable)

43 tests, 100% coverage on all PR2 modules, 0.15s wall-clock.

Author: maxisbey

src/mcp/shared/jsonrpc_dispatcher.py

@@ -359,8 +359,10 @@ def _dispatch(
                 self._dispatch_notification(msg, metadata, on_notify, sender_ctx)
             case JSONRPCResponse():
                 self._resolve_pending(msg.id, msg.result)
-            case JSONRPCError():
+            case JSONRPCError():  # pragma: no branch
                 # `id` may be None per JSON-RPC (parse error before id known).
+                # The match is exhaustive over JSONRPCMessage; the no-match arc
+                # on this final case is unreachable.
                 self._resolve_pending(msg.id, msg.error)
 
     def _dispatch_request(
@@ -537,7 +539,5 @@ async def _write_error(self, request_id: RequestId, error: ErrorData) -> None:
     async def _cancel_outbound(self, request_id: RequestId, reason: str) -> None:
         try:
             await self.notify("notifications/cancelled", {"requestId": request_id, "reason": reason})
-        except anyio.BrokenResourceError:
-            pass
-        except anyio.ClosedResourceError:
+        except (anyio.BrokenResourceError, anyio.ClosedResourceError):
             pass