fix: address review — fail loud on non-video bodies, dedupe form plumbing, doc parity

- videoMIME no longer hard-falls-back to video/mp4: a 2xx body that is
  neither declared nor sniffable as video (JSON job envelope, HTML error
  page) is now an APIError instead of a 'successful' corrupt clip.
- Resolution rides the wire as width/height AND the OpenAI-style size
  string, so either upstream convention honors an explicit request.
- writeFormFields + mimeFromContentType shared helpers replace the
  copied multipart loop (audio.go/video.go) and Content-Type branch.
- ADR-0019 indexed in docs/adr/README.md; README gains the videogen
  section + support-matrix mention (docs-parity rule).

Co-Authored-By: Claude Fable 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01AXQxVhXBw8PwFAtsVrXSmj

provider/llamaswap/video_test.go

@@ -179,6 +179,9 @@ func TestVideoModelRequiresBaseURL(t *testing.T) {
 }
 
 func TestVideoMIME(t *testing.T) {
+	// A minimal ISO-BMFF prefix http.DetectContentType sniffs as video/mp4
+	// (the "mp4" brand prefix must appear inside the declared ftyp box).
+	mp4Magic := append([]byte{0, 0, 0, 20}, []byte("ftypmp42\x00\x00\x00\x00mp42")...)
 	cases := []struct {
 		contentType string
 		data        []byte
@@ -186,12 +189,36 @@ func TestVideoMIME(t *testing.T) {
 	}{
 		{"video/webm", []byte("x"), "video/webm"},
 		{"video/mp4; charset=binary", []byte("x"), "video/mp4"},
-		{"application/octet-stream", []byte("x"), "video/mp4"},
-		{"", []byte("x"), "video/mp4"},
+		{"application/octet-stream", mp4Magic, "video/mp4"},
+		// Neither declared nor sniffable as video → "" (Generate errors).
+		{"application/octet-stream", []byte(`{"id":"job-1"}`), ""},
+		{"", []byte("x"), ""},
 	}
 	for _, tc := range cases {
 		if got := videoMIME(tc.contentType, tc.data); got != tc.want {
-			t.Errorf("videoMIME(%q) = %q, want %q", tc.contentType, got, tc.want)
+			t.Errorf("videoMIME(%q, %q) = %q, want %q", tc.contentType, tc.data, got, tc.want)
 		}
 	}
 }
+
+func TestVideoGenerateNonVideoBodyErrors(t *testing.T) {
+	// A stock async /v1/videos handler mounted at the sync path (or an HTML
+	// error page behind a proxy) answers 200 with a non-video body — that
+	// must be an error, never a "successful" garbage clip.
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"id":"job-1","status":"queued"}`))
+	}))
+	defer srv.Close()
+
+	p := New(WithBaseURL(srv.URL), WithHTTPClient(srv.Client()))
+	vm, _ := p.VideoModel("videogen-wan")
+	_, err := vm.Generate(context.Background(), videogen.Request{Prompt: "x"})
+	var apiErr *llm.APIError
+	if !errors.As(err, &apiErr) {
+		t.Fatalf("err = %v, want *llm.APIError for non-video 2xx body", err)
+	}
+	if !strings.Contains(apiErr.Message, "not a video") {
+		t.Errorf("message = %q, want mention of non-video body", apiErr.Message)
+	}
+}