chore(deps): Update litellm requirement from >=1.86.2 to >=1.91.0

Updates the requirements on [litellm](https://github.com/BerriAI/litellm) to permit the latest version.
- [Release notes](https://github.com/BerriAI/litellm/releases)
- [Commits](https://github.com/BerriAI/litellm/compare/v1.86.2...v1.91.0)

---
updated-dependencies:
- dependency-name: litellm
  dependency-version: 1.91.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -9,7 +9,7 @@ psutil
 fire
 fuzzywuzzy
 openai
-litellm>=1.86.2  # to support `from litellm import get_valid_models`
+litellm>=1.91.0  # to support `from litellm import get_valid_models`
 aiohttp>=3.14.0  # CVE-2026-22815, CVE-2026-34515, CVE-2026-34516, CVE-2026-34525; >=3.13.4 due to litellm==1.83.14 exact pin
 azure.identity
 pyarrow