fix(deps): DeprecationWarning on HTTP_413_REQUEST_ENTITY_TOO_LARGE (#693)

# Description

In [This commit that landed in
v0.48.0](Kludex/starlette@40a8147)
starlette changed constants to conform to
[RFC9110](https://www.rfc-editor.org/rfc/rfc9110#name-413-content-too-large).

It's the only [one
affected](Kludex/starlette@40a8147#diff-966c562bff3b11736a75544f42526c942c185b9fe9eb4b7f5a0dabbeca874fa7R180)
that I could find in this codebase.


- [x] Follow the [`CONTRIBUTING`
Guide](https://github.com/a2aproject/a2a-python/blob/main/CONTRIBUTING.md).
- [x] Make your Pull Request title in the
<https://www.conventionalcommits.org/> specification.
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- `feat!:`, or `fix!:`, `refactor!:`, etc., which represent a breaking
change (indicated by the `!`) and will result in a SemVer major.
- [x] Ensure the tests and linter pass (Run `bash scripts/format.sh`
from the repository root to format)
- [x] Appropriate docs were updated (if necessary)

# Tests
```zsh
uv sync --all-groups
```
Before and after: confirmed that this is fixed.
```zsh
uv run python -W error::DeprecationWarning -c "from a2a.server.apps.jsonrpc.fastapi_app import *"   
```
Confirmed that tests worked with
```zsh
uv run pytest
```

# Other
uv.lock currently has 

```toml
[[package]]
name = "starlette"
version = "0.50.0"
```
So I don't think the dependency pinning is a breaking change.

---------

Co-authored-by: Holt Skinner <13262395+holtskinner@users.noreply.github.com>

Author: SrzStephen

src/a2a/server/apps/jsonrpc/jsonrpc_app.py

@@ -64,7 +64,14 @@
     from starlette.exceptions import HTTPException
     from starlette.requests import Request
     from starlette.responses import JSONResponse, Response
-    from starlette.status import HTTP_413_REQUEST_ENTITY_TOO_LARGE
+
+    try:
+        # Starlette v0.48.0
+        from starlette.status import HTTP_413_CONTENT_TOO_LARGE
+    except ImportError:
+        from starlette.status import (  # type: ignore[no-redef]
+            HTTP_413_REQUEST_ENTITY_TOO_LARGE as HTTP_413_CONTENT_TOO_LARGE,
+        )
 
     _package_starlette_installed = True
 else:
@@ -76,7 +83,14 @@
         from starlette.exceptions import HTTPException
         from starlette.requests import Request
         from starlette.responses import JSONResponse, Response
-        from starlette.status import HTTP_413_REQUEST_ENTITY_TOO_LARGE
+
+        try:
+            # Starlette v0.48.0
+            from starlette.status import HTTP_413_CONTENT_TOO_LARGE
+        except ImportError:
+            from starlette.status import (
+                HTTP_413_REQUEST_ENTITY_TOO_LARGE as HTTP_413_CONTENT_TOO_LARGE,
+            )
 
         _package_starlette_installed = True
     except ImportError:
@@ -90,7 +104,7 @@
         Request = Any
         JSONResponse = Any
         Response = Any
-        HTTP_413_REQUEST_ENTITY_TOO_LARGE = Any
+        HTTP_413_CONTENT_TOO_LARGE = Any
 
 
 class StarletteUserProxy(A2AUser):
@@ -381,7 +395,7 @@ async def _handle_requests(self, request: Request) -> Response:  # noqa: PLR0911
                 None, A2AError(root=JSONParseError(message=str(e)))
             )
         except HTTPException as e:
-            if e.status_code == HTTP_413_REQUEST_ENTITY_TOO_LARGE:
+            if e.status_code == HTTP_413_CONTENT_TOO_LARGE:
                 return self._generate_error_response(
                     request_id,
                     A2AError(