fix(videogen): distinct FILENAMES for the two keyframes, not just distinct field names

Caught while writing the receiving end. Distinct multipart field names are not
sufficient: backends stage an uploaded frame under a name derived from the
FILENAME, and our own ComfyUI shim posts to /upload/image with overwrite=true.
Both parts were sending initImageFilename(mime) — literally "frame.png" for
each — so the second upload would have clobbered the first and BOTH keyframe
inputs would have resolved to the same stored image.

The failure mode is the worst kind: a clip pinned at both ends to the same
frame renders cleanly, returns 200, and looks like the feature not working
rather than like a bug. Nothing upstream or downstream would report a fault.

writeImagePart now takes the filename stem (frame / frame_last), and the test
asserts the two arrive under different filenames.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01PLjgrxvHjm1sJgUu9zBPH9

provider/llamaswap/video_test.go

@@ -233,19 +233,22 @@ func TestVideoGenerateNonVideoBodyErrors(t *testing.T) {
 // breaking. Asserting the names is what pins it.
 func TestVideoGenerateSendsBothKeyframes(t *testing.T) {
 	var gotFirst, gotLast []byte
+	var firstName, lastName string
 	var sawLastPart bool
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		if err := r.ParseMultipartForm(32 << 20); err != nil {
 			t.Errorf("parse form: %v", err)
 			return
 		}
-		if f, _, err := r.FormFile("input_reference"); err == nil {
+		if f, hdr, err := r.FormFile("input_reference"); err == nil {
 			gotFirst, _ = io.ReadAll(f)
+			firstName = hdr.Filename
 			f.Close()
 		}
-		if f, _, err := r.FormFile("input_reference_last"); err == nil {
+		if f, hdr, err := r.FormFile("input_reference_last"); err == nil {
 			sawLastPart = true
 			gotLast, _ = io.ReadAll(f)
+			lastName = hdr.Filename
 			f.Close()
 		}
 		w.Header().Set("Content-Type", "video/mp4")
@@ -283,6 +286,18 @@ func TestVideoGenerateSendsBothKeyframes(t *testing.T) {
 	if string(gotFirst) == string(gotLast) {
 		t.Error("both parts carry identical bytes — the frames are being aliased")
 	}
+	// DISTINCT FILENAMES, not just distinct field names. Backends stage an
+	// uploaded frame under a name derived from the filename (our ComfyUI shim
+	// posts to /upload/image with overwrite=true), so two parts sharing
+	// "frame.png" would have the second clobber the first and BOTH keyframes
+	// would resolve to the same stored image — a clip pinned at both ends to
+	// the same frame, rendering cleanly with nothing reporting a fault.
+	if firstName == "" || lastName == "" {
+		t.Fatalf("filenames = %q / %q, want both set", firstName, lastName)
+	}
+	if firstName == lastName {
+		t.Errorf("both parts use filename %q — the second upload would clobber the first", firstName)
+	}
 }
 
 // LastImage alone (no InitImage) is a legitimate request: pin the destination