Drop GraphMatrixT export and make GraphPostprocessor a PEP 695 type alias

[selmanozleyen]

Follow-up to #1222. That PR deliberately left out two breaking changes (see its "Deliberately not done" section); now that the graph-builder API is still experimental, this applies both.

Changes

• Drop GraphMatrixT from the public API. Removed from __all__ in squidpy.gr and squidpy.gr.neighbors, plus the two autosummary entries in docs/api.md. It was only exported to back the GraphPostprocessor alias.
• Convert GraphPostprocessor to a PEP 695 type alias whose type parameter is scoped to the alias itself. This removes the now-unused module-level GraphMatrixT = TypeVar(...) and the TypeVar import.

Breaking changes

• from squidpy.gr import GraphMatrixT (and from squidpy.gr.neighbors import GraphMatrixT) no longer works — the name is gone.
• GraphPostprocessor's runtime type changes from a plain typing Callable form to TypeAliasType. Type-level usage (GraphPostprocessor[csr_matrix], annotations) is unchanged.

The GraphBuilder[CoordT, GraphMatrixT] class param and SpatialNeighborsResult[GraphMatrixT] are independent PEP 695 params and are unaffected.

Verification

• ruff check src/squidpy/gr/ clean.
• Runtime asserts: type(GraphPostprocessor).__name__ == "TypeAliasType", GraphPostprocessor[csr_matrix] still subscriptable, GraphMatrixT absent from both __all__s and no longer an attribute of squidpy.gr.neighbors.
• tests/graph/test_spatial_neighbors.py: 50 passed.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.53%. Comparing base (9f9d500) to head (1768fa3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1223      +/-   ##
==========================================
- Coverage   76.53%   76.53%   -0.01%     
==========================================
  Files          63       63              
  Lines        9070     9069       -1     
  Branches     1523     1523              
==========================================
- Hits         6942     6941       -1     
  Misses       1544     1544              
  Partials      584      584              

Files with missing lines	Coverage Δ
src/squidpy/gr/neighbors.py	90.74% <100.00%> (-0.05%)	⬇️

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