Verify translation difference formulas

[inducer]

Subsumes #260 and #261.

[Copilot]

Pull request overview

Adds new coefficient-translation verification tests to the sumpy/test suite, consolidating prior work from #260 and #261 to validate “difference formula” relationships between compressed and full expansion translations.

Changes:

• Add an L2L coefficient-difference verification test across several kernels.
• Add an M2M coefficient-difference verification test across several kernels.
• Introduce shared symbolic-to-numeric helper utilities and update the basedpyright baseline accordingly.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 6 comments.

File	Description
sumpy/test/test_m2m_coeffs.py	New M2M translation coefficient difference verification test.
sumpy/test/test_l2l_coeffs.py	New L2L translation coefficient difference verification test.
sumpy/test/coeff_test_tools.py	New shared helpers for symbolic substitution and numeric matvec/matrix building.
.basedpyright/baseline.json	Baseline update to account for new files / type-checking findings.

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[inducer]

Good to go from my end, subject to license approval.