fix unmix state when changing value

[kecnry]

Description

This pull request fixes a bug in plot options unmix_state when the value is being changed by unmixing (which as far as I can tell only happens from calls within the RGB presets and only shows as a bug when the original tab selection is the last tab that is iterated during the RGB presets call and so hard to cover with a regression test).

Change log entry

• 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)

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• 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.
• Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• Do the proposed changes follow the STScI Style Guides?
• Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• Did the CI pass? If not, are the failures related?
• 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.
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.80%. Comparing base (f4923fd) to head (d79d837).
Report is 83 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3296   +/-   ##
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  Coverage   88.80%   88.80%           
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  Files         125      125           
  Lines       19019    19021    +2     
=======================================
+ Hits        16890    16892    +2     
  Misses       2129     2129           

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[gibsongreen]

It was super intricate to reproduce the bug in the first place, code changes are straightforward so I'm good with merging the PR as is if you are!

[pllim]

If new_value == self.value, why is _on_value_changed needed?

[kecnry]

to propagate to all the linked states (this happens when clicking to unmix the state, self.value is the nominal value (of the first linked state) and we want to propagate that to all other linked glue states)

[kecnry]

(see the description above for the scenario where this makes a difference, probably could do the same by forcing a send_state, but that would then call _on_value_changed twice)

[pllim]

Code looks like code.