fix(slurm): parse structured errors from non-2xx Slurm API responses

Replace ErrorResponse with SlurmAPIError that extracts structured errors/warnings from JSON body when Slurm returns non-2xx (e.g. 404 with valid JSON). Add IsNotFound helper for fallback logic.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

internal/slurm/client_test.go

@@ -137,11 +137,11 @@ func TestClient_ErrorHandling(t *testing.T) {
 		t.Fatal("expected error for 500 response")
 	}
 
-	errorResp, ok := err.(*ErrorResponse)
+	errorResp, ok := err.(*SlurmAPIError)
 	if !ok {
-		t.Fatalf("expected *ErrorResponse, got %T", err)
+		t.Fatalf("expected *SlurmAPIError, got %T", err)
 	}
-	if errorResp.Response.StatusCode != 500 {
-		t.Errorf("expected status 500, got %d", errorResp.Response.StatusCode)
+	if errorResp.StatusCode != 500 {
+		t.Errorf("expected status 500, got %d", errorResp.StatusCode)
 	}
 }