Canonicalize tests to @safetestset for module isolation

[ChrisRackauckas-Claude]

What

Converts each independent test unit to run in its own module via @safetestset, matching the canonical SciML test structure (isolation between tests + world-age safety). Behavior-preserving: same tests run, same assertions, same GROUP dispatch.

Core (test/runtests.jl)

The Core group was a single monolithic @testset "OrgMaintenanceScripts.jl" wrapping 4 inline @testsets plus 7 include(...) calls, all relying on imports leaked into Main from the top of runtests.jl.

• The 4 inline units (Version bumping, Project file handling, Repository processing, Basic functionality legacy) become inline @safetestset blocks, each carrying its own using lines (OrgMaintenanceScripts, Test, plus TOML where used).
• Each per-file unit becomes @safetestset "Name" begin include("x.jl") end. The included files are made self-contained: the stale # Note: ... already loaded by runtests.jl comments are replaced with explicit using lines. Two files additionally need using Dates (they call now() / DateTime at top level), and one needs using TOML (calls TOML.print). These previously worked only via Main-leaked imports.
• The already-commented-out multiprocess_testing include is preserved (still disabled), shown in @safetestset form for consistency.

QA (test/qa.jl)

The two plain @testset units (Aqua, JET) are extracted into qa_aqua.jl / qa_jet.jl and wrapped as @safetestset ... include(...). The include() form is required for the JET unit because @test_opt is a package macro that must resolve after using JET runs (top-level statements in the included file macroexpand one at a time, unlike an inline @safetestset body). @test_broken lines preserved.

Deps

SafeTestsets added to Project.toml [extras] / [targets].test and [compat] SafeTestsets = "0.1, 1"; same UUID + compat added to test/qa/Project.toml (QA runs in its own activated env). using SafeTestsets added at the top of runtests.jl.

Verification

Ran locally on Julia 1.11: GROUP=Core Pkg.test() passes. All 11 converted units pass:
Version bumping 5/5, Project file handling 6/6, Repository processing 8/8, Basic functionality (legacy) 2/2, Formatting Functions 9/9, Minimum Version Fixer 25/25, Version Check Finder 23/23, Invalidation Analysis 36/36, Import Timing Analysis 56/56, Explicit Imports Fixer 10/10, Documentation Cleanup 41/41.

(The "Analysis failed" log lines from the invalidation/import-timing tests are pre-existing expected behavior: those tests deliberately run analysis on a mock package and assert the failure path returns -1 — unrelated to this change.)

Ignore until reviewed by @ChrisRackauckas.

[ChrisRackauckas-Claude]

Superseded by the v1.2 folder conversion on sciml-testing-rollout (#63).