QA: Aqua/JET findings marked @test_broken pending fix

As part of the grouped-tests CI conversion (branch grouped-tests-ci), the QA test group runs Aqua + JET. Two genuine Aqua findings and one genuine JET finding currently fail the QA group, so they have been wrapped as @test_broken in test/qa/qa.jl to keep the grouped-tests QA group green and allow the PR to merge. They should be fixed and un-broken.

Findings (Julia 1.11, Aqua 0.8.16, JET 0.9.20):

1. Aqua.test_all deps_compat (extras) — MathML does not declare a [compat] entry for the following [extras]: Pkg [44cfe95a-1eb2-52ea-b672-e2afdf69b78f]. Fix: add a [compat] bound for Pkg (e.g. Pkg = "1"), or drop Pkg from [extras]/[targets] if it is no longer needed by the test suite.

2. Aqua.test_all piracies — possible type piracy: three methods defined on EzXML.Node in MathML:

   • children(n::EzXML.Node) @ src/utils.jl:77
   • printnode(io::IO, node::EzXML.Node) @ src/utils.jl:78
   • nodetype(::EzXML.Node) @ src/utils.jl:79
children/nodetype/printnode are owned by AbstractTrees, and the only argument types are owned by EzXML — neither is owned by MathML, hence piracy. Fix options: move these AbstractTrees interface methods upstream into EzXML (or an EzXML-AbstractTrees glue package), or wrap EzXML.Node in a MathML-owned wrapper type before defining the interface.

3. JET.test_package (report_package) — 1 possible error: no matching method found mathml_to_nums(::EzXML.Node), called from mathml_to_nums(xml::EzXML.Document) @ src/utils.jl:20 (return mathml_to_nums(doc_root) where doc_root::EzXML.Node). The docstring for mathml_to_nums claims an EzXML.Node method exists, but no such method is defined. Fix: define mathml_to_nums(::EzXML.Node) (the function the Document/AbstractString methods are supposed to dispatch into).

The remaining Aqua sub-checks (ambiguities, unbound type params, undefined exports, project-extras comparison, stale deps, julia/deps/weakdeps compat, persistent tasks) pass; only the two Aqua checks and the one JET check above are wrapped broken.

Ignore until reviewed by @ChrisRackauckas.

Start with test/qa/qa.jl and inspect the @test_broken wrappers around the Aqua and JET checks. Read src/utils.jl at lines 20 and 77-79, then run the QA group with Julia 1.11, Aqua 0.8.16, and JET 0.9.20; done means the three findings are fixed, the wrappers are removed, and the QA group remains green.