fix: add overloads to AsyncSession.send() for stream-dependent return type (#335)

## Summary
                        
`AsyncSession.send()` returns `Response | AsyncResponse` without
narrowing based on the `stream` parameter, unlike `request()`, `get()`,
`post()`, and other public methods which already have `@overload` stubs.
                                           
## Fix
Adds `@typing.overload` stubs to `AsyncSession.send()` so the return
type is narrowed based on `stream`:
  - `stream=True` → `AsyncResponse`
  - default → `Response`

Author: Andarius

src/niquests/async_session.py

@@ -450,6 +450,12 @@ def get_adapter(self, url: str) -> AsyncBaseAdapter:  # type: ignore[override]
         # Nothing matches :-/
         raise InvalidSchema(f"No connection adapters were found for {url!r}{additional_hint}")
 
+    @typing.overload  # type: ignore[override]
+    async def send(self, request: PreparedRequest, *, stream: Literal[True], **kwargs: typing.Any) -> AsyncResponse: ...
+
+    @typing.overload  # type: ignore[override]
+    async def send(self, request: PreparedRequest, **kwargs: typing.Any) -> Response: ...
+
     async def send(  # type: ignore[override]
         self, request: PreparedRequest, **kwargs: typing.Any
     ) -> Response | AsyncResponse:  # type: ignore[override]