feat: cancel_job over REST (Database::cancel_job + remote impl + pyo3 + python)

Exposes the existing server-side CANCEL JOB (CoordinatorCatalog::cancel_job)
as a REST-backed SDK method: Database trait default NotSupported,
RemoteDatabase POSTs /v1/job/{id}/cancel, pyo3 binding, sync+async python
wrappers. Best-effort: a missing job returns false, not an error. Mock-HTTP
unit test in test_derived_compute_routes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

python/src/connection.rs

@@ -510,6 +510,13 @@ impl Connection {
         })
     }
 
+    pub fn cancel_job(self_: PyRef<'_, Self>, job_id: String) -> PyResult<Bound<'_, PyAny>> {
+        let inner = self_.get_inner()?.clone();
+        future_into_py(self_.py(), async move {
+            inner.cancel_job(&job_id).await.infer_error()
+        })
+    }
+
     #[pyo3(signature = (cur_name, new_name, cur_namespace_path=None, new_namespace_path=None))]
     pub fn rename_table(
         self_: PyRef<'_, Self>,