feat(models): response models for the upload-library methods

[atirna]

Summary

• get_library_upload_songs, get_library_upload_artist and get_library_upload_album now return pydantic response models (UploadSong, UploadAlbum) deriving from the YTMusicModel base landed in feat(models): add pydantic and the YTMusicModel dict-compatible base #1004
• parsing lives in ytmusicapi/parsers/uploads.py: parse_uploaded_items constructs UploadSong directly, and a new parse_upload_album assembles the album header plus tracks
• docstrings point at the models instead of repeating the JSON shape
• get_continuations / get_continuation_contents take a TypeVar so a parse func returning models typechecks; the dict parse funcs everywhere else are unchanged

Closes #987

Why

uploaded items are similar to but not the same as regular songs: they carry an entityId and lack several fields YTM-hosted songs have (isAvailable, videoType, setVideoId). since the song model from #983 and the shared leaf models from #981 are not merged yet, these are upload-specific models for now. the leaf shapes (Thumbnail, ArtistRef, AlbumRef) deliberately match what #981 plans, so re-homing them onto the shared models later is mostly a rename.

one behavioural note for 2.0 consumers: with the #307 key-presence semantics, a field the API did not return is present with value None instead of being absent. "album" in song is True even when the song has no album column, and song["album"] returns None instead of raising. dict-style access (song["title"], song.get("entityId")) keeps working through the mapping protocol, so code like the existing tests/mixins/test_uploads.py flows is unaffected.

Verification

• pytest -o addopts='' tests/parsers tests/models -q: 103 passed. new tests in tests/parsers/test_uploads.py cover: full song parse, missing fixedColumns / album column / thumbnails staying present as None, unknown likeStatus falling back to INDIFFERENT, menu-less items skipped, a continuation page parsed through the model parse func, and album duration_seconds summed over tracks (4:15 => 255, two tracks => 510)
• mypy (strict, repo config), clean
• ruff check . and ruff format --check .: clean
• the live tests in tests/mixins/test_uploads.py are unchanged and run in CI with the repo credentials