fix: validate --format flag and use cobra output writer

Address Copilot review feedback: reject unsupported --format values
before making the network request instead of silently falling back to
the default output, and print JSON via cmd.Println so the output
respects the command's configured writer.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Rahul Vishwakarma <rahulvs2809@gmail.com>

Author: manzil-infinity180

cmd/archivistactl/cmd/search.go

@@ -44,6 +44,10 @@ Digests are expected to be in the form algorithm:digest, for instance: sha256:45
 		return nil
 	},
 	RunE: func(cmd *cobra.Command, args []string) error {
+		if format != "table" && format != "json" {
+			return fmt.Errorf("unsupported format %q: supported formats are table, json", format)
+		}
+
 		algo, digest, err := validateDigestString(args[0])
 		if err != nil {
 			return err
@@ -59,7 +63,7 @@ Digests are expected to be in the form algorithm:digest, for instance: sha256:45
 			if err != nil {
 				return fmt.Errorf("failed to marshal results to JSON: %w", err)
 			}
-			fmt.Println(string(jsonData))
+			cmd.Println(string(jsonData))
 			return nil
 		}