fix(model): reject null/empty YAML mapping keys in ParseBytes

A bare '?' in YAML is an explicit-key indicator for a null key, which
produced a job with an empty name (p.Jobs[""]) — a parser invariant
violation caught by FuzzParseBytes.

ParseBytes now returns an error when keyNode.Value is empty.
Failing corpus entry retained as a seed so CI exercises this path.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/model/parser.go

@@ -57,6 +57,9 @@ func ParseBytes(data []byte) (*Pipeline, error) {
 		keyNode := root.Content[i]
 		valNode := root.Content[i+1]
 		key := keyNode.Value
+		if key == "" {
+			return nil, fmt.Errorf("job name cannot be empty (null or missing YAML key)")
+		}
 		if ReservedKeys[key] {
 			continue
 		}