Fix PartsCount

[peterjan]

When retrieving an object by part number, the parts count was incorrectly returned as 0 for non-multipart objects, and the part query was missing filename, sia_object, and cached_at fields which prevented serving part data from disk or Sia.

[Copilot]

Pull request overview

This PR fixes object-part retrieval so that part-based GET/HEAD requests can correctly serve data for both regular and multipart objects, including cases where the data must be read from disk or from Sia.

Changes:

• Adjust SQLite object retrieval to validate part numbers correctly for non-multipart objects and to include additional object fields when querying a specific part.
• Update Sia backend part responses to return a correct PartsCount for non-multipart objects when a partNumber is specified.
• Add regression tests covering non-multipart objects requested with a part number.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
sia/persist/sqlite/objects_test.go	Extends DB-level coverage for part-number fetches, including ensuring SiaObject is returned.
sia/persist/sqlite/objects.go	Fixes part-number logic and expands the part query to include fields needed to serve from disk/Sia.
sia/objects.go	Adjusts PartsCount returned for part-number GET/HEAD on non-multipart objects.
s3/objects_test.go	Adds regression test for part-number requests on non-multipart objects.
.gitignore	Ignores GeoLite2 DB file.

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