Remove stale version checks and fix soft scope warning

[ChrisRackauckas-Claude]

Summary

• Remove @static if isdefined(Base, :@assume_effects) fallback in src/ArrayInterface.jl — @assume_effects has been available since Julia 1.8, and compat requires 1.10+
• Remove VERSION >= v"1.7" guard in test/core.jl (always true, dead Val(true) QR code path)
• Remove VERSION >= v"1.9-" guard in test/core.jl (always true, cholesky_instance test now runs unconditionally)
• Remove VERSION >= v"1.8-" guard in ext/ArrayInterfaceCUDAExt.jl (always true, dead lu(A; check=false) fallback)
• Fix soft scope warning for B variable in test/bandedmatrices.jl by adding local B

No deprecation warnings were found in ArrayInterface.jl or its test dependencies on Julia 1.12.4 (verified with Pkg.test(; julia_args=["--depwarn=error"])). This PR cleans up stale version-gated code that can never execute given the Julia 1.10+ compat requirement.

Test plan

• All 303 tests pass locally with Pkg.test(; julia_args=["--depwarn=error"])
• Soft scope warning in bandedmatrices.jl is resolved
• CI passes

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.35%. Comparing base (4b9414a) to head (4938f58).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
ext/ArrayInterfaceCUDAExt.jl	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #478      +/-   ##
==========================================
+ Coverage   61.13%   61.35%   +0.21%     
==========================================
  Files          13       13              
  Lines         579      577       -2     
==========================================
  Hits          354      354              
+ Misses        225      223       -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.