Assert the StaticArrays extension actually loaded in QA

[ChrisRackauckas-Claude]

Follow-up to #263, which was merged before this guard was pushed.

Why

The extension scan that #263 turned on can be silently lost again.
ExplicitImports skips an extension whose module does not exist:

ext_mod = Base.get_extension(mod, Symbol(ext))
ext_mod === nothing && continue

That skip is silent — every check then reports a clean pass. So if a future
change breaks ExponentialUtilitiesStaticArraysExt's precompilation, or the
using StaticArrays line in test/qa/qa.jl gets dropped, QA goes green while
extension coverage quietly falls back to zero, which is exactly the state #263
just fixed.

What changed

Assert the extension module exists rather than trusting a green run_qa:

@testset "Extensions loaded" begin
    @test Base.get_extension(
        ExponentialUtilities, :ExponentialUtilitiesStaticArraysExt
    ) !== nothing
end

Verified locally that this actually discriminates rather than being vacuously
true: in a session with only using ExponentialUtilities, Base.get_extension
returns nothing; after using StaticArrays it returns the module.

Local verification

GROUP=QA julia --project=. -e 'using Pkg; Pkg.test()' on this branch:

Test Summary: | Pass  Broken  Total     Time
QA/qa.jl      |   22       2     24  3m30.7s
     Testing ExponentialUtilities tests passed

That is 22 passing versus 21 on master, the difference being the new guard. The
2 broken are the pre-existing broken = true markers in the AllocCheck testset,
untouched here. No package source is changed by this PR, only test/qa/qa.jl.

---

Please ignore this PR until it has been reviewed by @ChrisRackauckas.