Route sublibrary groups from the root test harness

[ChrisRackauckas-Claude]

Ignore this PR until it has been reviewed by @ChrisRackauckas.

What changed and why

Pass the repository's lib directory to SciMLTesting's root run_tests call. This enables the existing monorepo router to resolve prefixed groups such as ModelingToolkitBase_Extensions and run the requested sublibrary tests.

This is a one-line test-harness correction. It changes no package source, public API, dependency, or version.

Root cause

The harness conversion in d647d61 from #4610 adopted SciMLTesting but omitted lib_dir from the root call.

Its parent accepts the prefixed command but silently performs zero tests:

0.000040 seconds (59 allocations)

Current master exposes the omission as a hard routing error instead. The existing report is #4997.

Failing before

On clean current master 38d91d81dcd77319a70abb93882ba6da13510600:

GROUP=ModelingToolkitBase_Extensions julia +1.12 --project=. -e 'using Pkg; Pkg.test()'

exits before any owning test runs:

run_tests: group entry must be a function or vector, got Nothing
_group_spec(... Nothing)
Testing ModelingToolkit tests failed

Passing after

The identical root command now dispatches to the actual sublibrary suite and exits 0:

HomotopyContinuation | 76/76
LabelledArrays        | 18/18
BifurcationKit        | 7/7
Despecialized MTKParameters AD | 2/2
Testing ModelingToolkitBase tests passed
Testing ModelingToolkit tests passed

The root QA group also passes:

GROUP=QA
QA | 52/52
Testing ModelingToolkit tests passed

Whole-repository Runic, spelling over the diff, and git diff --check pass.

Not verified

Other prefixed sublibrary groups were not each run locally; this change supplies the generic lib_dir consumed by the shared router. Docs were not run because only the test harness changed. Julia LTS/prerelease were not repeated locally.

[ChrisRackauckas-Claude]

The current CI failures are outside this PR's one-line QA-routing change:

• Typos fails on pre-existing docs text in docs/src/internals/mtkcompile.md: three real misspellings (machanism, remaning, expeceted) and two legitimate bibliography abbreviations (Comput.) that typos flags.
• Catalyst downstream failures are excluded from this QA effort and report a separate ModelingToolkitBase.SymbolCache issue.

The routing change itself passes Runic and the completed non-Catalyst downstream/downgrade lanes. I am keeping typo cleanup separate so this PR remains focused.

[ChrisRackauckas-Claude]

Baseline CI audit for the unrelated Spell Check and docs build failures:

• Spell Check reproduces unchanged on clean master with five findings in docs/src/internals/mtkcompile.md. Formal bisect: aae673d from docs: add internal docs page for mtkcompile #4993. A mechanical-only local fix has exact fail-before/pass-after, root QA 52/52, and whole-repo Runic green. It is being held until the independent docs prerequisite lands so its first CI revision is not knowingly red.
• The docs build reproduces on clean master because the public @docs block still names ModelingToolkitBase.hessian_sparsity after that stale binding was removed in 63f684c. The focused fix is draft Remove stale non-public sparsity bindings from docs #5002; its identical full LTS docs command fails before and passes after.

Neither failure is caused by this PR's focused diff.

[ChrisRackauckas-Claude]

Rebased onto current master and pushed the updated draft branch. Audit remains scoped to test-group routing: the root GROUP=ModelingToolkitBase_Extensions run passed (Testing ModelingToolkitBase tests passed, Testing ModelingToolkit tests passed); Runic, typos, and diff checks pass. No Catalyst or unrelated behavior was changed. Existing CI jobs are still running; the prior Catalyst/downstream failures remain outside this PR.