Fix/#2663 Add UI checks to vtkF3DImguiObservers to prevent segfaults

[Samarthi]

Describe your changes

• Add UI render checks to ImguiObservers
• Test behaviour of Observer in the absence of explicit renders

Issue ticket number and link if any

#2663

Checklist for finalizing the PR

• [x] I have performed a self-review of my code
• [x] I have added tests for new features and bugfixes
• [ ] I have added documentation for new features
• [ ] If it is a modifying the libf3d API, I have updated bindings
• [ ] If it is a modifying the .github/workflows/versions.json, I have updated docker_timestamp

Continuous integration

Please write a comment to run CI, eg: \ci fast. See here for more info.

[Samarthi]

\ci full

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 96.53%. Comparing base (2103357) to head (2fed866). :warning: Report is 15 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2706      +/-   ##
==========================================
+ Coverage   96.52%   96.53%   +0.01%     
==========================================
  Files         141      141              
  Lines       12478    12535      +57     
==========================================
+ Hits        12044    12101      +57     
  Misses        434      434              

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• :package: JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[mwestphal]

Thanks for the contribution @Samarthi !