Update SciMLTesting QA compat

[ChrisRackauckas-Claude]

Summary

• update SciMLTesting compat entries to exactly 2.1 in the relevant root/test QA environments
• migrate existing QA to SciMLTesting 2.1 public API docs behavior where needed

Ignore this draft PR until reviewed by @ChrisRackauckas.

Validation

• ran local QA for this package as noted in the agent report
• ran TOML parse checks for edited Project.toml files
• ran Runic.jl on changed Julia files where applicable
• grepped for api_docs = false, docstrings_broken, Base.Docs, missing_docstrings, and stale SciMLTesting 1.x compat pins