Fix REST endpoints for adding tags

[nscuro]

Description

Fixes REST endpoints for adding tags.

The /tag/{name}/project endpoint did not add the provided tag to projects which already had tags assigned to them.

The /tag/{name}/policy and /tag/{name}/notificationRule endpoints replaced existing tags with the provided tag, which is not the intended behavior. Since only one tag can be provided via path parameter, the tag should be added to existing tags, not replace them.

Addressed Issue

Fixes #4539

Additional Details

N/A

Checklist

• I have read and understand the contributing guidelines
• This PR fixes a defect, and I have provided tests to verify that the fix is effective
• This PR implements an enhancement, and I have provided tests to verify that it works as intended
• This PR introduces changes to the database model, and I have added corresponding update logic
• This PR introduces new or alters existing behavior, and I have updated the documentation accordingly

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.00% (target: -1.00%)	✅ 90.00% (target: 70.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (bcd2651)	23013	18284	79.45%
Head commit (cb4590f)	23023 (+10)	18293 (+9)	79.46% (+0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#4541)	40	36	90.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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