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_test.go

@@ -88,6 +88,12 @@ func TestParseBytes_EdgeCases(t *testing.T) {
 	if err == nil {
 		t.Error("wrong field type: expected error from ParseBytes (stage must be string)")
 	}
+
+	// Null/empty YAML key (e.g. bare "?"): job name cannot be empty.
+	_, err = ParseBytes([]byte("?"))
+	if err == nil {
+		t.Error("null key: expected error from ParseBytes (job name cannot be empty)")
+	}
 }
 
 // TestParse_ParseBytesError exercises the Parse → ParseBytes error path (line 18).