Update httpx requirement from >=0.25.0 to >=0.28.1 in /src/a2a

Updates the requirements on [httpx](https://github.com/encode/httpx) to permit the latest version.
- [Release notes](https://github.com/encode/httpx/releases)
- [Changelog](https://github.com/encode/httpx/blob/master/CHANGELOG.md)
- [Commits](encode/httpx@0.25.0...0.28.1)

---
updated-dependencies:
- dependency-name: httpx
  dependency-version: 0.28.1
  dependency-type: direct:production
...

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

Author: dependabot[bot]

src/a2a/requirements_a2a.txt

@@ -1,9 +1,9 @@
-fastapi>=0.104.0
+fastapi>=0.104.0
 uvicorn[standard]>=0.24.0
 starlette>=0.27.0
 pydantic>=2.5.0
 aiofiles>=23.2.0
-httpx>=0.25.0
+httpx>=0.28.1
 psutil>=5.9.0
 prometheus-client>=0.19.0
 gunicorn>=21.2.0