Refactor starlette imports in utility modules

Ensures [http-server] optional dependency imports are handled within
try-except in utility modules:
- server/request_handlers/rest_handler.py
- and utils/error_handlers.py
Also, removes duplicate imports from `starlette` in `rest_adapter.py`.

Author: darkhaniop

src/a2a/server/apps/rest/rest_adapter.py

@@ -4,10 +4,6 @@
 from collections.abc import AsyncIterable, AsyncIterator, Awaitable, Callable
 from typing import TYPE_CHECKING, Any
 
-from sse_starlette.sse import EventSourceResponse
-from starlette.requests import Request
-from starlette.responses import JSONResponse, Response
-
 
 if TYPE_CHECKING:
     from sse_starlette.sse import EventSourceResponse

src/a2a/server/request_handlers/rest_handler.py

@@ -1,10 +1,19 @@
 import logging
 
 from collections.abc import AsyncIterable, AsyncIterator
-from typing import Any
+from typing import TYPE_CHECKING, Any
 
 from google.protobuf.json_format import MessageToDict, MessageToJson, Parse
-from starlette.requests import Request
+
+
+if TYPE_CHECKING:
+    from starlette.requests import Request
+else:
+    try:
+        from starlette.requests import Request
+    except ImportError:
+        Request = Any
+
 
 from a2a.grpc import a2a_pb2
 from a2a.server.context import ServerCallContext

src/a2a/utils/error_handlers.py

@@ -2,9 +2,18 @@
 import logging
 
 from collections.abc import Awaitable, Callable, Coroutine
-from typing import Any
+from typing import TYPE_CHECKING, Any
+
+
+if TYPE_CHECKING:
+    from starlette.responses import JSONResponse, Response
+else:
+    try:
+        from starlette.responses import JSONResponse, Response
+    except ImportError:
+        JSONResponse = Any
+        Response = Any
 
-from starlette.responses import JSONResponse, Response
 
 from a2a._base import A2ABaseModel
 from a2a.types import (