feat(track): add optional per-track session parameter (#96)

Originally authored by panitan103 — adds optional session parameter to Track class allowing services to pass per-track sessions with different headers/cookies/auth.

Changes for dev branch integration:
- Fix type hints to support both requests.Session and RnetSession
- Fix session fallback in dl.py: track.session or service.session
- Remove redundant `session or None` assignment

Co-Authored-By: panitan103 <panitan103@users.noreply.github.com>

unshackle/commands/dl.py

@@ -2069,7 +2069,7 @@ class dl:
                             (
                                 pool.submit(
                                     track.download,
-                                    session=service.session,
+                                    session=track.session or service.session,
                                     prepare_drm=partial(
                                         partial(self.prepare_drm, table=download_table),
                                         track=track,

unshackle/core/tracks/track.py

@@ -45,6 +45,7 @@ class Track:
         name: Optional[str] = None,
         drm: Optional[Iterable[DRM_T]] = None,
         edition: Optional[str] = None,
+        session: Optional[Union[Session, "RnetSession"]] = None,
         downloader: Optional[Callable] = None,
         downloader_args: Optional[dict] = None,
         from_file: Optional[Path] = None,
@@ -99,6 +100,7 @@ class Track:
         self.name = name
         self.drm = drm
         self.edition: list[str] = [edition] if isinstance(edition, str) else (edition or [])
+        self.session = session
         self.downloader = downloader
         self.downloader_args = downloader_args
         self.from_file = from_file
@@ -180,7 +182,7 @@ class Track:
 
     def download(
         self,
-        session: Session,
+        session: Union[Session, "RnetSession"],
         prepare_drm: partial,
         max_workers: Optional[int] = None,
         progress: Optional[partial] = None,