Add Media Field for Album and Test case to test it

[gcoppola19]

Fixes #6092

Add a media field for album, when a user runs beets fields, media will be listed for album. This way, media of album can be kept track of.

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• [x ] Tests.
  test_media_field.py

[sourcery-ai]

Hey there - I've reviewed your changes and they look great!

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location> `test/test_media_field.py:4-11` </location>
<code_context>
+from beets.library import Item
+from beets import library
+
+def test_album_media_field(tmp_path):
+    lib = library.Library(path=str(tmp_path / "library.db"),
+                          directory=str(tmp_path / "music"))
+
+    item = Item(title="Test Song", album="Test Album", media="Vinyl")
+    album = lib.add_album([item])   
+
+    assert album.media == "Vinyl"
\ No newline at end of file
</code_context>

<issue_to_address>
**suggestion (testing):** Consider adding tests for edge cases and error conditions for the 'media' field.

Additional tests for cases where 'media' is missing, None, an empty string, or a non-string value would improve coverage and help identify potential issues.
</issue_to_address>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.34%. Comparing base (beda6fc) to head (9663371).
⚠️ Report is 18 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6151      +/-   ##
==========================================
+ Coverage   67.23%   67.34%   +0.10%     
==========================================
  Files         120      136      +16     
  Lines       18440    18492      +52     
  Branches     3134     3134              
==========================================
+ Hits        12398    12453      +55     
+ Misses       5372     5370       -2     
+ Partials      670      669       -1     

Files with missing lines	Coverage Δ
beets/library/models.py	87.17% <ø> (ø)

... and 21 files with indirect coverage changes

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