Bump httpx from 0.27.2 to 0.28.1 in /backend

Bumps [httpx](https://github.com/encode/httpx) from 0.27.2 to 0.28.1.
- [Release notes](https://github.com/encode/httpx/releases)
- [Changelog](https://github.com/encode/httpx/blob/master/CHANGELOG.md)
- [Commits](encode/httpx@0.27.2...0.28.1)

---
updated-dependencies:
- dependency-name: httpx
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

backend/pyproject.toml

@@ -15,7 +15,7 @@ dependencies = [
     "camel-converter[pydantic]==4.0.1",
     "fastapi==0.115.6",
     "httptools==0.6.4",
-    "httpx==0.27.2",
+    "httpx==0.28.1",
     "langchain==0.3.13",
     "langchain-openai==0.2.14",
     "langgraph==0.2.60",