Migrate QA to SciMLTesting 2.4

[ChrisRackauckas-Claude]

Summary

• Require SciMLTesting 2.4 in the package and QA environments, relying on its strict default public API checks.
• Remove the package-local dynamic public-doc scanner and redundant QA overrides; retain only two fixed stdlib concrete-type ExplicitImports exceptions.
• Improve definition-site docs for callback storage types and fix scalar sum accumulation to match the documented API, with focused tests.

Verification

• Strict QA with SciMLTesting 2.4.0: 18 / 18 passed.
• Core test group passed, including new scalar Gaussian and Gauss-Kronrod sum tests.
• Runic and git diff --check passed.
• A source-only docs run completed cross-reference and document checks but timed out during Documenter HTML writing after one hour.

Ignore until reviewed by @ChrisRackauckas.