Bump pydantic-settings from 2.1.0 to 2.12.0 in /reference-apps/fastapi (#14)

Bumps [pydantic-settings](https://github.com/pydantic/pydantic-settings) from 2.1.0 to 2.12.0.
- [Release notes](https://github.com/pydantic/pydantic-settings/releases)
- [Commits](pydantic/pydantic-settings@v2.1.0...v2.12.0)

---
updated-dependencies:
- dependency-name: pydantic-settings
  dependency-version: 2.12.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

reference-apps/fastapi/requirements.txt

@@ -1,6 +1,6 @@
 fastapi==0.121.1
 uvicorn[standard]==0.38.0
-pydantic-settings==2.1.0
+pydantic-settings==2.12.0
 slowapi==0.1.9  # Rate limiting
 pybreaker==1.0.1  # Circuit breaker