Address review feedback on list-dependents

- Tag the usage code fence as bash so it satisfies markdownlint MD040.
- Reject unsupported --format values explicitly with a guard clause
  rather than silently falling back to JSON, so typos in CI or local
  use surface as errors.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: theletterf

docs/cli/docset/list-dependents.md

@@ -13,7 +13,7 @@ that would re-render, so the comment can point at them instead.
 
 ## Usage
 
-```
+```bash
 docs-builder list-dependents --files <paths> [options...] [-h|--help] [--version]
 ```
 

src/authoring/Elastic.Documentation.Refactor/Tracking/ListDependentsService.cs

@@ -37,6 +37,13 @@ public async Task<bool> ListDependents(
 			return false;
 		}
 
+		if (!format.Equals("json", StringComparison.OrdinalIgnoreCase) &&
+			!format.Equals("text", StringComparison.OrdinalIgnoreCase))
+		{
+			collector.EmitGlobalError($"Unsupported format '{format}'. Expected 'json' or 'text'.");
+			return false;
+		}
+
 		var context = new BuildContext(collector, fs, fs, configurationContext, ExportOptions.MetadataOnly, path, null);
 		var set = new DocumentationSet(context, logFactory, NoopCrossLinkResolver.Instance);