feat(slurmdb): add DiagService

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

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

internal/slurm/slurmdb_diag_test.go

@@ -0,0 +1,51 @@
+package slurm
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+func TestSlurmdbDiagService_GetDiag(t *testing.T) {
+	mux := http.NewServeMux()
+	mux.HandleFunc("/slurmdb/v0.0.40/diag", func(w http.ResponseWriter, r *http.Request) {
+		testMethod(t, r, "GET")
+		fmt.Fprint(w, `{"statistics": {"time_start": 1700000000}}`)
+	})
+	server := httptest.NewServer(mux)
+	defer server.Close()
+
+	client, _ := NewClient(server.URL, nil)
+	resp, _, err := client.SlurmdbDiag.GetDiag(context.Background())
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if resp == nil {
+		t.Fatal("expected non-nil response")
+	}
+	if resp.Statistics == nil {
+		t.Fatal("expected non-nil statistics")
+	}
+}
+
+func TestSlurmdbDiagService_GetDiag_Error(t *testing.T) {
+	mux := http.NewServeMux()
+	mux.HandleFunc("/slurmdb/v0.0.40/diag", func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+		fmt.Fprint(w, `{"errors": [{"error": "internal error"}]}`)
+	})
+	server := httptest.NewServer(mux)
+	defer server.Close()
+
+	client, _ := NewClient(server.URL, nil)
+	_, _, err := client.SlurmdbDiag.GetDiag(context.Background())
+	if err == nil {
+		t.Fatal("expected error for 500 response")
+	}
+	if !strings.Contains(err.Error(), "500") {
+		t.Errorf("expected error to contain 500, got %v", err)
+	}
+}