feat: add scoped settings context manager

[PragnyaKhandelwal]

Summary

• Add a scoped settings context manager for temporary perf/debug/inference overrides
• Add a defaults accessor to create a fresh AppSettings
• Add a regression test to verify settings are restored after the scoped block exits

Issue resolved by this Pull Request:
Resolves #3417

Checklist:

• Documentation has been updated, if necessary.
• Examples have been added, if necessary.
• Tests have been added, if necessary.

[github-actions]

✅ DCO Check Passed

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