BUG: Fix wrong Subset unit when created in 2D spectrum viewer

[pllim]

Description

This pull request is to address wrong Subset unit when it is extracted from Subset created in Specviz2d on the 2D spectrum viewer. Glue itself does not attach unit natively to Subset, so that means Jdaviz made wrong assumptions in the extraction code. Bug ticket blamed it to the Export plugin but the actual bug has nothing to do with the Export plugin, thought the plugin did expose the bug when user extracted Subset that way but you don't need the plugin to reproduce the bug.

Change log entry

• [x] Is a change log needed? If yes, is it added to CHANGES.rst? If you want to avoid merge conflicts, list the proposed change log here for review and add to CHANGES.rst before merge. If no, maintainer should add a no-changelog-entry-needed label.

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• [x] Are two approvals required? Branch protection rule does not check for the second approval. If a second approval is not necessary, please apply the trivial label.
• [x] Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• [x] Do the proposed changes follow the STScI Style Guides?
• [x] Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• [x] Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• [x] Did the CI pass? If not, are the failures related?
• [x] Is a milestone set? Set this to bugfix milestone if this is a bug fix and needs to be released ASAP; otherwise, set this to the next major release milestone. Bugfix milestone also needs an accompanying backport label.
• [x] After merge, any internal documentations need updating (e.g., JIRA, Innerspace)? 🐱

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 66.66667% with 5 lines in your changes missing coverage. Please review.

Project coverage is 88.44%. Comparing base (f880d55) to head (eafb391). Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
jdaviz/app.py	66.66%	5 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3201 +/- ## ========================================== - Coverage 88.46% 88.44% -0.02% ========================================== Files 125 125 Lines 18668 18676 +8 ========================================== + Hits 16514 16518 +4 - Misses 2154 2158 +4 ```

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[pllim]

@kecnry , I think I addressed your comment.

[pllim]

Actually I am not going to backport this because it is unlikely the backport will be clean due to unit conversion stuff.

[pllim]

Thanks, all!