test(types): regression test that ContextHeader is hidden from JSON

Locks in two invariants exposed during the integration audit:
- ContextHeader on types.Chunk must never leak into the JSON
  representation served to API clients (json:"-" + gorm:"-").
- types.ParsedChunk.EmbeddingContent prepends ContextHeader with a
  blank line separator and falls back to plain Content when the
  header is unset.

https://claude.ai/code/session_01XADhx6mtu2ZYW3DE9Lun6k

internal/types/chunk_contextheader_test.go

@@ -0,0 +1,35 @@
+package types
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+)
+
+func TestChunkContextHeaderHiddenInJSON(t *testing.T) {
+	c := Chunk{
+		ID:            "test",
+		Content:       "body",
+		ContextHeader: "# Heading",
+	}
+	b, err := json.Marshal(c)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(string(b), "Heading") || strings.Contains(string(b), "context_header") {
+		t.Errorf("ContextHeader leaked into JSON: %s", string(b))
+	}
+}
+
+func TestParsedChunkEmbeddingContent(t *testing.T) {
+	pc := ParsedChunk{Content: "body", ContextHeader: "# H"}
+	got := pc.EmbeddingContent()
+	want := "# H\n\nbody"
+	if got != want {
+		t.Errorf("EmbeddingContent mismatch: got %q, want %q", got, want)
+	}
+	pc2 := ParsedChunk{Content: "only body"}
+	if pc2.EmbeddingContent() != "only body" {
+		t.Errorf("EmbeddingContent without header should equal Content")
+	}
+}