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/fuzz_test.go

@@ -26,6 +26,7 @@ func FuzzParseBytes(f *testing.F) {
 		[]byte("job:\n  stage: test\n  image:\n    name: golang:1.21\n    entrypoint: ['']\n  parallel:\n    matrix:\n      - PLATFORM: [linux, darwin]\n  script: go build\n"),
 		[]byte("default:\n  retry: 2\n  timeout: 1h30m\nvariables:\n  ENV: production\nstages: [build, test, deploy]\n"),
 		[]byte("&anchor\n  script: [echo ok]\njob:\n  <<: *anchor\n  stage: build\n"),
+		[]byte("?"), // null/empty YAML key — must error, not produce an empty-named job
 	}
 	for _, s := range seeds {
 		f.Add(s)

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
 		}

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).

internal/model/testdata/fuzz/FuzzParseBytes/c18522695dec4f60

@@ -0,0 +1,2 @@
+go test fuzz v1
+[]byte("?")