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.go

@@ -0,0 +1,19 @@
+package slurm
+
+import "context"
+
+// GetDiag returns slurmdbd diagnostics information.
+func (s *SlurmdbDiagService) GetDiag(ctx context.Context) (*OpenapiSlurmdbdStatsResp, *Response, error) {
+	path := "slurmdb/v0.0.40/diag"
+	req, err := s.client.NewRequest("GET", path, nil)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	var result OpenapiSlurmdbdStatsResp
+	resp, err := s.client.Do(ctx, req, &result)
+	if err != nil {
+		return nil, resp, err
+	}
+	return &result, resp, nil
+}