fix(rest): send historyLength=0 (avoid falsy omission) (#480)

**What** Ensure `history_length=0` results in `?historyLength=0` being sent by the REST client. **Why** `0` was treated as falsy → the param was omitted → servers returned the full history. **How** Replace truthy check with explicit `is not None`. **Ref** - Permalink: https://github.com/a2aproject/a2a-python/blob/4a4b4a92d7fce6db02f397753ce3ec0dfcd8a597/src/a2a/client/transports/rest.py#L217 Fixes #479 Co-authored-by: asechi_tadaki <asechi_tadaki@infordio.co.jp>

Author: TadakiAsechi

src/a2a/client/transports/rest.py

@@ -214,7 +214,7 @@ async def get_task(
  response_data = await self._send_get_request(
  f'/v1/tasks/{request.id}',
  {'historyLength': str(request.history_length)}
- if request.history_length
+ if request.history_length is not None
  else {},
  modified_kwargs,
  )