Update pydantic requirement from >=2.5.0 to >=2.13.3 in /src/a2a

Updates the requirements on [pydantic](https://github.com/pydantic/pydantic) to permit the latest version.
- [Release notes](https://github.com/pydantic/pydantic/releases)
- [Changelog](https://github.com/pydantic/pydantic/blob/main/HISTORY.md)
- [Commits](pydantic/pydantic@v2.5.0...v2.13.3)

---
updated-dependencies:
- dependency-name: pydantic
  dependency-version: 2.13.3
  dependency-type: direct:production
...

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

Author: dependabot[bot]

src/a2a/requirements_a2a.txt

@@ -1,7 +1,7 @@
-fastapi>=0.104.0
+fastapi>=0.104.0
 uvicorn[standard]>=0.24.0
 starlette>=0.27.0
-pydantic>=2.5.0
+pydantic>=2.13.3
 aiofiles>=23.2.0
 httpx>=0.25.0
 psutil>=5.9.0