fix(transcode): ignore Content-Length from non-2xx HEAD responses

resolve_file_size() trusted Content-Length unconditionally, so a 405
(or any 4xx/5xx) HEAD response with a short error body (e.g. 18-byte
"Method Not Allowed") set _file_size to that tiny value. The probe
stage then clamped header_size to 18 bytes, logged "Header too small",
and the transcode pipeline returned 404.

Only read Content-Length from HEAD when status < 400; otherwise fall
through to the existing Range: bytes=0-0 GET branch which reads the
real size from Content-Range.

Fixes: upstreams like AIOStreams /api/v1/debrid/playback/ that 405 on HEAD.

Closes #276

Author: mhdzumair

mediaflow_proxy/remuxer/media_source.py

@@ -198,7 +198,7 @@ async def resolve_file_size(self) -> int:
                 proxy=proxy_url,
                 allow_redirects=True,
             ) as resp:
-                cl = resp.headers.get("content-length")
+                cl = resp.headers.get("content-length") if resp.status < 400 else None
                 if cl:
                     self._file_size = int(cl)
                 else: