fix: skip blank/whitespace-only lines in receive loop

`_receive_loop` only checked for EOF (`b""`), but blank lines like
`b"\n"`, `b" \n"`, or `b"\r\n"` are truthy and fell through to
`json.loads()`, causing `JSONDecodeError: Expecting value: line 1
column 2 (char 1)`.  This is common on WSL where Windows-native agent
subprocesses emit `\r\n` blanks, or any CLI that prints empty lines to
stdout alongside JSON-RPC frames.

Strip each line and skip it when empty before attempting JSON parse.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: aorumbayev

src/acp/connection.py

@@ -151,6 +151,9 @@ async def _receive_loop(self) -> None:
                 line = await self._reader.readline()
                 if not line:
                     break
+                line = line.strip()
+                if not line:
+                    continue
                 try:
                     message: dict[str, Any] = json.loads(line)
                 except Exception:

tests/test_rpc.py

@@ -282,6 +282,24 @@ async def test_ignore_invalid_messages(connect, server):
         await asyncio.wait_for(server.client_reader.readline(), timeout=0.1)
 
 
+@pytest.mark.asyncio
+async def test_blank_lines_skipped_without_error(connect, server):
+    """Blank/whitespace-only lines on the wire must not cause JSONDecodeError."""
+    connect(connect_agent=True, connect_client=False)
+
+    # Send blank lines, then a valid request
+    for noise in [b"\n", b"  \n", b"\r\n"]:
+        server.client_writer.write(noise)
+    req = {"jsonrpc": "2.0", "id": 1, "method": "initialize", "params": {"protocolVersion": 1}}
+    server.client_writer.write((json.dumps(req) + "\n").encode())
+    await server.client_writer.drain()
+
+    line = await asyncio.wait_for(server.client_reader.readline(), timeout=1)
+    resp = json.loads(line)
+    assert resp["id"] == 1
+    assert "result" in resp
+
+
 class _ExampleAgent(Agent):
     __test__ = False