fix: clarify TestPlugin_StepSchemaCoverage doc comment to match what it actually tests

The comment said "every step type advertised in StepTypes()" but the test
reads from plugin.json's stepTypes array, not the Go StepTypes() method.

Agent-Logs-Url: https://github.com/GoCodeAlone/workflow-plugin-data-engineering/sessions/7e0eb14e-5aa9-4dc7-a3b0-a6ae661d980c

Co-authored-by: intel352 <77607+intel352@users.noreply.github.com>

Author: Copilot

internal/plugin_test.go

@@ -594,9 +594,10 @@ func repoRoot(t *testing.T) string {
 	return filepath.Dir(filepath.Dir(file))
 }
 
-// TestPlugin_StepSchemaCoverage verifies that every step type advertised in
-// StepTypes() has a corresponding entry in the stepSchemas field of plugin.json.
-// This ensures strict contract descriptors are present for all steps.
+// TestPlugin_StepSchemaCoverage verifies that every step type listed in the
+// stepTypes array of plugin.json has a corresponding entry in the stepSchemas
+// array of the same file, and that no orphan stepSchemas entries exist.
+// This ensures strict contract descriptors are present for all advertised steps.
 func TestPlugin_StepSchemaCoverage(t *testing.T) {
 	root := repoRoot(t)
 	data, err := os.ReadFile(filepath.Join(root, "plugin.json"))