[python] Enforce if-not-exists semantics for append/registration

[johnkerl]

Issue and/or context:

There are two use-cases for registration:

• The SOMA Experiment does not exist, and there are one or more AnnData objects to be registered and then ingested.
  • This user intent is signaled by passing experiment_uri=None to the registrar
• The SOMA Experiment does exist, and there are one or more AnnData objects to be co-registered with the existing experiment, and then the AnnData objects are to be ingested
  • This user intent is signaled by passing a non-empty experiment_uri to the registrar

Due to an unforunate implementation bug (in the code touched by this PR) these two use-cases were getting conflated.

Changes:

• Raise an exception if the experiment_uri passed to the registrar is not None. This is a non-breaking change, since the existing docstrings already make it sound like what this PR is doing was already the case.
• Make the docstrings even clearer
• Unit-test the behavior

Notes for Reviewer:

[sc-44602]

[shortcut-integration]

This pull request has been linked to Shortcut Story #44602: AnnData registration silently fails when unauthenticated.

[codecov]

Codecov Report

Merging #2384 (9286b1e) into main (f0c483d) will increase coverage by 24.55%.
The diff coverage is 100.00%.

❗ Current head 9286b1e differs from pull request most recent head dd67eca. Consider uploading reports for the commit dd67eca to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2384       +/-   ##
===========================================
+ Coverage   66.04%   90.60%   +24.55%     
===========================================
  Files         144       37      -107     
  Lines       13005     3895     -9110     
  Branches      511        0      -511     
===========================================
- Hits         8589     3529     -5060     
+ Misses       4316      366     -3950     
+ Partials      100        0      -100     

Flag	Coverage Δ
libtiledbsoma	?
python	90.60% <100.00%> (-0.01%)	⬇️
r	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
python_api	90.60% <100.00%> (-0.01%)	⬇️
libtiledbsoma	∅ <ø> (∅)

[johnkerl]

[sc-44675]

[shortcut-integration]

This pull request has been linked to Shortcut Story #44675: tiledbsoma 1.9.4.