Assert the extension actually loaded in QA

`Base.get_extension` returning `nothing` makes ExplicitImports skip the
extension silently, so a broken extension would leave QA green while dropping
extension coverage back to zero. Check the module exists before `run_qa`.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>

Author: ChrisRackauckas

test/qa/qa.jl

@@ -6,6 +6,12 @@ using Test
 # load every weakdep here to bring the extensions into the QA scan.
 using PrettyTables
 
+# ExplicitImports silently skips an extension that fails to load, so assert the
+# extension modules actually exist rather than trusting a green run_qa.
+@testset "Extensions loaded" begin
+    @test Base.get_extension(SymbolicIndexingInterface, :SymbolicIndexingInterfacePrettyTablesExt) !== nothing
+end
+
 # ExplicitImports per-check ignore-lists: each entry is a dependency name that is
 # genuinely required but is neither exported nor declared `public` by its owner
 # package, and has no public alternative to switch to.