[python] Fix readback of some older-style metadata

[johnkerl]

Issue and/or context: sc-62798

Bug introduced as of TileDB-SOMA 1.15.4 via PR #3607 which was the backport to the release-1.15 branch of #3558.

Changes:

Metadata were raeding back incorrectly in the cases when their values' tdb_type was TILEDB_STRING_ASCII (11) rather than TILEDB_STRING_UTF8 (12).

Note, however, that I ran the following test today:

• Write arrays using TileDB-SOMA 1.14.5, 1.15.0, 1.15.1, 1.15.2, 1.15.3, 1.15.4, and 1.15.5, pip-install each version
• Outer-loop over those same versions, pip-installing each one
  • Inner-loop over the arrays from above
  • Try using version y to read metadata written with version x
  • In all cases I could not get a fail

Another test:

• I read various old arrays I'd created 1, 3, 6 months ago, and could not repro the bug

In short, this occurs for some arrays but not all arrays. (All arrays tested in-house read back fine, with tiledbsoma 1.15.5).

Notes for Reviewer:

Uncertain how to test, given failure to repro with any of my historical or current data.

One idea:

• Hack tiledbsoma (in sandbox) to write metadata with value-type TILEDB_STRING_ASCII
• Create some data
• Save that off in the repo as canned data
• Run unit tests on that

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.30%. Comparing base (a42d6b5) to head (a67d998).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3653      +/-   ##
==========================================
+ Coverage   86.25%   86.30%   +0.04%     
==========================================
  Files          55       55              
  Lines        6381     6381              
==========================================
+ Hits         5504     5507       +3     
+ Misses        877      874       -3     

Flag	Coverage Δ
python	86.30% <ø> (+0.04%)	⬆️

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

Components	Coverage Δ
python_api	86.30% <ø> (+0.04%)	⬆️
libtiledbsoma	∅ <ø> (∅)

[johnkerl]

I also validated that the alternative fix

         } else {
-            py::dtype value_type = tdb_to_np_dtype(tdb_type, 1);
+            py::dtype value_type = tdb_to_np_dtype(tdb_type, value_num);
             results[py::str(key)] = py::array(value_type, value_num, value)
                                         .attr("item")(0);
         }

resolves the reported symptom.

(Note that that 1 predates #3558.)

[johnkerl]

Either of these two lines fix the symptom. As noted in the description field, though, we need more unit-test coverage here.

[johnkerl]

#3654