MLM bugfixes

[jonas-hurst]

**Description:
Fixed some bugs in mlm extension (see changelog for details)

PR Checklist:

• Pre-commit hooks pass (run pre-commit run --all-files)
• Tests pass (run pytest)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[jonas-hurst]

I continued working on my (synced) fork, so this PR also lists commits that have already been merged a few weeks ago... Sorry for the mess...

[codecov]

Codecov Report

❌ Patch coverage is 98.53480% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.51%. Comparing base (918c629) to head (0965157).

Files with missing lines	Patch %	Lines
pystac/extensions/mlm.py	98.52%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1556      +/-   ##
==========================================
+ Coverage   92.38%   92.51%   +0.13%     
==========================================
  Files          55       55              
  Lines        8374     8575     +201     
  Branches      965     1037      +72     
==========================================
+ Hits         7736     7933     +197     
- Misses        453      454       +1     
- Partials      185      188       +3     

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[gadomski]

Thanks for the contribution and fixes! I can review the code, but I don't have much visibility into how the MLM extension works. Could you possibly suggest two or three folks that might be able to review this PR from for content and usability?

[jsignell]

It might also make sense to cherry-pick the new commits onto a fresh branch off origin:main so that the diff is easier to parse.

[jonas-hurst]

continuing this in #1571