Make the buffer regression test reach the code it covers

A WebM source with format=MP4 never entered the remux path: "mp4" is
absent from the source demuxer's name, so the pre-existing
reuse_streams guard already forced a transcode and the test passed
against an unfixed tree. A mov source carrying pcm_u8 keeps
reuse_streams true and is refused by the mp4 muxer, which is the case
the test was meant to describe, and it is the only test that reaches
the BytesIO truncate.

Also decode the outputs rather than trusting stream headers, encode the
audio-fallback source as mpeg4 so the h264 assertion distinguishes a
re-encode from a copy, stop pinning the mp4 demuxer's full alias list,
and skip rather than error when a build lacks an encoder the sources
need.

tests-unit/comfy_api_test/video_types_test.py

@@ -727,9 +727,32 @@ def test_save_to_mp4_writes_metadata_before_media(video_components, tmp_path):
         assert data.index(b"moov") < data.index(b"mdat")
 
 
+def require_encoder(name):
+    try:
+        av.codec.Codec(name, "w")
+    except av.codec.codec.UnknownCodecError:
+        pytest.skip(f"{name} encoder not available in this PyAV build")
+
+
+def decoded_counts(container):
+    video = sum(len(packet.decode()) for packet in container.demux(container.streams.video[0]))
+    audio = 0
+    if container.streams.audio:
+        container.seek(0)
+        audio = sum(
+            frame.samples
+            for packet in container.demux(container.streams.audio[0])
+            for frame in packet.decode()
+        )
+    return video, audio
+
+
 def create_matroska_source(
     tmp_path, video_codec="libvpx", audio_codec=None, frames=6, fps=10, container_format="webm"
 ):
+    require_encoder(video_codec)
+    if audio_codec is not None:
+        require_encoder(audio_codec)
     path = str(tmp_path / f"source_{video_codec}.{container_format}")
     with av.open(path, mode="w", format=container_format) as container:
         video_stream = container.add_stream(video_codec, rate=fps)
@@ -768,13 +791,14 @@ def test_save_to_auto_transcodes_codec_the_container_cannot_store(tmp_path):
     VideoFromFile(source).save_to(destination)
 
     with av.open(destination) as container:
-        assert container.format.name == "mov,mp4,m4a,3gp,3g2,mj2"
+        assert "mp4" in container.format.name.split(",")
         assert container.streams.video[0].codec.name == "h264"
+        assert decoded_counts(container)[0] == 6
 
 
 def test_save_to_auto_transcodes_when_only_audio_is_unsupported(tmp_path):
     source = create_matroska_source(
-        tmp_path, video_codec="libx264", audio_codec="pcm_u8", container_format="matroska"
+        tmp_path, video_codec="mpeg4", audio_codec="pcm_u8", container_format="matroska"
     )
     destination = str(tmp_path / "saved.mp4")
 
@@ -783,6 +807,9 @@ def test_save_to_auto_transcodes_when_only_audio_is_unsupported(tmp_path):
     with av.open(destination) as container:
         assert container.streams.video[0].codec.name == "h264"
         assert container.streams.audio[0].codec.name == "aac"
+        video_frames, audio_samples = decoded_counts(container)
+        assert video_frames == 6
+        assert audio_samples > 0
 
 
 def test_save_to_auto_still_remuxes_a_compatible_codec(tmp_path):
@@ -793,17 +820,23 @@ def test_save_to_auto_still_remuxes_a_compatible_codec(tmp_path):
 
     with av.open(destination) as container:
         assert container.streams.video[0].codec.name == "vp9"
+        assert decoded_counts(container)[0] == 6
 
 
 def test_save_to_buffer_transcodes_codec_the_container_cannot_store(tmp_path):
-    source = create_matroska_source(tmp_path)
+    source = create_matroska_source(
+        tmp_path, video_codec="mpeg4", audio_codec="pcm_u8", container_format="mov"
+    )
     buffer = io.BytesIO()
+    buffer.write(b"\x00" * 4096)
 
     VideoFromFile(source).save_to(buffer, format=VideoContainer.MP4)
 
     buffer.seek(0)
     with av.open(buffer) as container:
         assert container.streams.video[0].codec.name == "h264"
+        assert container.streams.audio[0].codec.name == "aac"
+        assert decoded_counts(container)[0] == 6
 
 
 def create_transcode_source(