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**Related issue:** Resolves#37092
# Checklist for submitter
If some of the following don't apply, delete the relevant line.
- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
## Testing
- [x] Added/updated automated tests
- [x] QA'd all new/changed functionality manually
<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit
* **New Features**
* Added per‑IP rate limiting to IdP metadata and SSO endpoints.
* Implemented TTL-backed in‑memory caching for IdP metadata responses to
reduce backend load.
* **Tests**
* Added tests covering metadata caching behavior, cache miss/error
handling, and content type preservation.
* Added tests validating rate limiting behavior across clients, bursts,
and proxy scenarios.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
Victor Lyuboslavsky
8f24773d2e