feat(concat_on_disk): support pairwise=True

[alejandro-publius]

• Closes Support pairwise for concat_on_disk #2414
• Tests added

concat_on_disk raised NotImplementedError for pairwise=True and never wrote the concatenation-axis pairwise group. It now writes .{obs,var}p as the block diagonal of each object's pairwise elements, matching concat_pairwise_mapping used by the in-memory concat: keys are intersected or unioned according to join, and an object lacking a key contributes an empty block.

The block diagonal is streamed one object at a time rather than assembled whole: row block i is that object's (n_i, n_i) element padded with zeros to (n_i, N) and appended to the output dataset, so only a single object's pairwise element is resident at once. The result dtype is resolved across all objects before the first block is written, since that write fixes the dtype of the dataset the remaining blocks are appended to; otherwise a leading empty stand-in block would pin the output to bool.

Tests check the result against concat for both file formats and both joins, cover objects that lack a key, and confirm .obsp is left out unless pairwise=True is passed.

This touches the same file as #2558 but is independent of it; whichever lands second will need a trivial rebase.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.87%. Comparing base (4ded337) to head (68ef855).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2636      +/-   ##
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- Coverage   87.64%   85.87%   -1.77%     
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  Files          49       49              
  Lines        8125     8151      +26     
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- Hits         7121     7000     -121     
- Misses       1004     1151     +147     

Files with missing lines	Coverage Δ
src/anndata/experimental/merge.py	90.33% <100.00%> (+1.85%)	⬆️

... and 8 files with indirect coverage changes