Fix sampled matrix integration over dimensional axes

[ChrisRackauckas-Claude]

This PR should be ignored until reviewed by @ChrisRackauckas.

Summary

• Fix the sampled matrix fast path to allocate its accumulator using the weight * sample result type instead of eltype(data).
• Add a Unitful regression test for SampledIntegralProblem(data, axis; dim = 2) with a dimensional axis.
• Add Unitful as a test-only dependency through the main package test target.

Fixes #341.

Tests

• Reproduced the original failure on the unmodified branch with Julia 1.12.6: DimensionError: 0.0 and 0.5 m are not dimensionally compatible at src/sampled.jl:79.
• GROUP=Core timeout 3600 /home/crackauc/.juliaup/bin/julia +1.12 --project=. -e 'using Pkg; Pkg.test()'
  • Core/sampled_tests.jl: 30 passed
  • Core/interface_tests.jl: 871 passed
  • Core/inf_integral_tests.jl: 746 passed
  • overall: Testing Integrals tests passed
• GROUP=AD timeout 7200 /home/crackauc/.juliaup/bin/julia +1.10 --project=. -e 'using Pkg; Pkg.test()'
  • AD/derivative_tests.jl: 2005 passed, 1176 broken
  • AD/nested_ad_tests.jl: 6 passed
  • overall: Testing Integrals tests passed
• Runic v1.7.0: /home/crackauc/.juliaup/bin/julia +1.12 --project=.runic -e 'using Runic; exit(Runic.main(ARGS))' -- --check .