Enhance fidelity testing and benchmarks for mathematical models

[christianulson]

Summary

• What changed in code?
• Which user-visible or maintainer-visible behavior changed?

Backlog Link

• Backlog item or section:
• Percentual updated in docs/features-backlog/index.md:
• Evidence checklist reference: docs/features-backlog/progress-update-checklist.md
• If no backlog update was needed, explain why:

Test Evidence

• New or updated test files:
• Providers covered:
• Command used for validation:
• Result:

Scope And Gaps

• Known limitation kept intentionally:
• Provider-specific differences still open:
• Risk of documentation/code drift, if any:

Checklist

• Code change is linked to at least one test change or an explicit justification
• Backlog/doc status was updated, or the omission is justified above
• Provider coverage affected by this PR is stated explicitly
• Validation evidence is recorded above