[Bug]: Adding tags to multiple entries fails if tag is present on one

[CyanVoxel]

Checklist

• I am using an up-to-date version.
• I have read the documentation.
• I have searched existing issues.

TagStudio Version

Alpha 9.5.3+ (main branch)

Operating System & Version

macOS 15.5

Description

When selecting multiple entries that already contain one or more tags, when adding one of those present tags to the selection it will fail adding the tags to a portion of the entries. I believe this is due to #1028 exposing an oversight in the add_tags_to_entries() method, causing a regression of this behavior.

Expected Behavior

Tags should be added to every entry in the selection, but ignored if the tag is already present. Same logic goes for removing tags.

Steps to Reproduce

See description - try with a selection of 5 or more entries with one of them having the "Favorite" tag in the middle. Observe how one side of the selection does not receive the favorite tag when adding it.

Logs

N/A