Add a no-copy GJSON parse helper

gjson.ParseBytes copies the entire document before returning a result, which
is prohibitive for multi-megabyte requests. ParseGJSONBytesNoCopy mirrors the
existing GetGJSONBytesNoCopy contract: the returned result references the
caller's bytes, so callers must neither retain it nor mutate the buffer while
it is in use.

internal/util/gjson.go

@@ -14,3 +14,14 @@ func GetGJSONBytesNoCopy(data []byte, path string) gjson.Result {
 	}
 	return gjson.Get(unsafe.String(unsafe.SliceData(data), len(data)), path)
 }
+
+// ParseGJSONBytesNoCopy parses data into a GJSON result that references data
+// directly. gjson.ParseBytes copies the whole document, which is prohibitive
+// for multi-megabyte payloads. Callers must not retain the result or mutate
+// data while using it.
+func ParseGJSONBytesNoCopy(data []byte) gjson.Result {
+	if len(data) == 0 {
+		return gjson.Result{}
+	}
+	return gjson.Parse(unsafe.String(unsafe.SliceData(data), len(data)))
+}

internal/util/gjson_test.go

@@ -1,6 +1,9 @@
 package util
 
-import "testing"
+import (
+	"testing"
+	"unsafe"
+)
 
 func TestGetGJSONBytesNoCopy(t *testing.T) {
 	input := []byte(`{"request":{"contents":[{"role":"user"}]}}`)
@@ -15,3 +18,28 @@ func TestGetGJSONBytesNoCopyEmptyInput(t *testing.T) {
 		t.Fatalf("empty input result = %s, want missing", result.Raw)
 	}
 }
+
+func TestParseGJSONBytesNoCopy(t *testing.T) {
+	input := []byte(`{"request":{"contents":[{"role":"user"}]}}`)
+	root := ParseGJSONBytesNoCopy(input)
+	if !root.IsObject() || root.Get("request.contents.0.role").String() != "user" {
+		t.Fatalf("parsed root = %s, want user content array", root.Raw)
+	}
+}
+
+func TestParseGJSONBytesNoCopyReferencesInput(t *testing.T) {
+	input := []byte(`{"contents":[{"role":"user"}]}`)
+	root := ParseGJSONBytesNoCopy(input)
+	if len(root.Raw) != len(input) {
+		t.Fatalf("raw length = %d, want %d", len(root.Raw), len(input))
+	}
+	if unsafe.StringData(root.Raw) != unsafe.SliceData(input) {
+		t.Fatal("parsed result copied the input instead of referencing it")
+	}
+}
+
+func TestParseGJSONBytesNoCopyEmptyInput(t *testing.T) {
+	if result := ParseGJSONBytesNoCopy(nil); result.Exists() {
+		t.Fatalf("empty input result = %s, want missing", result.Raw)
+	}
+}