Add opt-in test group environment isolation

[ChrisRackauckas-Claude]

Summary

• add the public isolate_group_environments keyword to run_tests and run_everything
• run environment-backed All, Everything, and umbrella members in fresh Julia processes when opted in
• preserve direct named-group and default in-process behavior
• preserve the active project and Julia runtime flags in child processes
• bump SciMLTesting to 2.3.0 and document the public keyword

Motivation

Julia cannot unload a package after a test group activates a different environment. Aggregate suites could therefore combine a package module loaded from one environment with source resolved from another. This is reproducible when downstream groups require incompatible LinearSolve versions.

Implementation notes

The child re-enters the outer test entrypoint with the member group selected. Entry-point discovery prefers Base.PROGRAM_FILE and falls back to the external run_tests call site for eval-driven tests. Isolation is lazy and opt-in, so existing callers and directly selected groups retain their current behavior.

Tests performed locally

• Julia 1.10.11 full test suite: 299 passed, 0 failed
• Julia 1.12.6 full test suite: 314 passed, 0 failed
• synthetic same-UUID package fixture verifies default same-process behavior and opt-in fresh-process package selection
• wrapper re-entry, runtime flag propagation, and explicit test_dir from julia -e are covered
• whole-repository Runic inplace and check/diff: exit 0
• git diff --check: exit 0

Process

I reproduced the cross-environment package-version leak, reduced it to a same-process activation mismatch, implemented the opt-in dispatcher path, added regression fixtures, ran both supported local Julia test suites, and ran final repository-wide formatting and hygiene checks.

Review

Ignore this PR until reviewed by @ChrisRackauckas.

[ChrisRackauckas-Claude]

Scratchpad update for commit 48aa317:

1. Reproduced the failure as a loaded-module mismatch across per-group environments.
2. Chose an opt-in public keyword with a false default to preserve existing behavior.
3. Isolated only environment-backed aggregate members; direct named groups stay in process.
4. Added tests for same-UUID package replacement, default behavior, wrapper entrypoints, child Julia flags, and eval-driven direct groups.
5. Ran the full suite on Julia 1.10.11 (299 passed) and 1.12.6 (314 passed), followed by whole-tree Runic and diff hygiene checks.

The stacked ModelingToolkit consumer will remain separate and will depend on SciMLTesting 2.3.

[ChrisRackauckas-Claude]

Terminal CI follow-up: all 12 checks are green, including Julia LTS/current/prerelease on Linux, macOS, and Windows, plus downgrade, Runic, and spelling. This matches the local full-suite results of 299/299 on Julia 1.10 and 314/314 on Julia 1.12. No CI-specific failure appeared.

[ChrisRackauckas-Claude]

Follow-up audit for 13471de / head 830e813:

While reproducing the MTSL #495 IntegrationTest failure, I found that a fresh isolated group process fixed the loaded-module collision but could silently stop testing a path-developed upstream package. A standalone fresh GROUP=Downstream Pkg.test() was green (31/31), but its nested resolver selected registered ModelingToolkitStandardLibrary v2.29.4 instead of the clean-main checkout under test.

The follow-up preserves the active test sandbox's path-tracked packages in isolated children:

• the child records Pkg.dependencies() path sources before activating its group environment;
• those paths join the existing deduplicated single Pkg.develop call;
• a group's own [sources] entry remains authoritative;
• the behavior is gated to isolated-child reentry, so directly selected named groups are unchanged.

The regression develops an unregistered UpstreamProbe in the outer project and verifies that the child receives it, while a same-name root/group VersionProbe verifies group [sources] precedence.

Real-graph validation used the stacked MTK isolation change from SciML/ModelingToolkit.jl#4749, with clean-main MTSL and this SciMLTesting checkout developed by path. A narrowed GROUP=All aggregate spawned the actual isolated Downstream child. Its resolver explicitly retained path MTK, MTKBase, MTSL, SciMLTesting, and SciCompDSL; resolved LinearSolve 4.3.0 + ArnoldiMethod 0.4.0; successfully precompiled LinearSolveArnoldiMethodExt; and finished:

Downstream | 31 pass / 31 total | 6m51.5s
Testing ModelingToolkit tests passed

Final validation after merging current main normally (no force push):

• Julia 1.10.11: 330/330 passed
• Julia 1.12.6: 345/345 passed
• repository-wide Runic check passed
• all tracked TOML parsed; git diff --check passed

The merged additive API version remains 2.5.0. Every new commit includes the required Chris co-author trailer.