build(deps): bump black from 25.9.0 to 26.3.1 in /server

Bumps [black](https://github.com/psf/black) from 25.9.0 to 26.3.1.
- [Release notes](https://github.com/psf/black/releases)
- [Changelog](https://github.com/psf/black/blob/main/CHANGES.md)
- [Commits](psf/black@25.9.0...26.3.1)

---
updated-dependencies:
- dependency-name: black
  dependency-version: 26.3.1
  dependency-type: direct:production
...

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

Author: dependabot[bot]

server/pyproject.toml

@@ -7,7 +7,7 @@ dependencies = [
     "fastapi==0.119.0",
     "uvicorn==0.38.0",
     "mlx-lm==0.31.0",
-    "black==25.9.0",
+    "black==26.3.1",
     "openai-harmony==0.0.8",
     "openresponses-types"
 ]