build(deps): update fastapi requirement in /benchmark (#78)

Updates the requirements on [fastapi](https://github.com/fastapi/fastapi) to permit the latest version.
- [Release notes](https://github.com/fastapi/fastapi/releases)
- [Commits](fastapi/fastapi@0.139.2...0.140.13)

---
updated-dependencies:
- dependency-name: fastapi
  dependency-version: 0.140.13
  dependency-type: direct:production
...

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

Author: dependabot[bot]

benchmark/requirements.txt

@@ -6,5 +6,5 @@ pytest-timeout>=2.4.0
 # deps from completion() (litellm.responses.mcp → litellm.proxy.*). Declare them
 # until a litellm release makes that import lazy; litellm[proxy] is NOT an
 # option (its granian pin is unresolvable).
-fastapi>=0.139.2
+fastapi>=0.140.13
 orjson>=3.11.9