test(signal): cover SSE receive loop and the empty-phone start guard

Previously the SSE loop and the empty-phone-number short-circuit in start()
had zero coverage. Both now have tests: a fake httpx stream feeds canned
SSE lines, exercising the valid-frame, invalid-JSON, non-200, and
no-http-client paths; start() with an empty phone number is asserted to
return without entering the HTTP loop.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/channels/test_signal_channel.py

@@ -3,8 +3,9 @@
 from __future__ import annotations
 
 import asyncio
+from contextlib import asynccontextmanager
 from pathlib import Path
-from unittest.mock import AsyncMock
+from unittest.mock import AsyncMock, MagicMock
 
 import pytest
 
@@ -891,6 +892,112 @@ class TestHandleDataMessageGroup:
         assert ch._id_matches_account("new-bot-uuid")
 
 
+# ---------------------------------------------------------------------------
+# Lifecycle / SSE
+# ---------------------------------------------------------------------------
+
+
+class _FakeSSEResponse:
+    """Minimal stand-in for httpx Response under stream()."""
+
+    def __init__(self, lines: list[str], status_code: int = 200) -> None:
+        self.status_code = status_code
+        self._lines = lines
+
+    async def aiter_lines(self):
+        for line in self._lines:
+            yield line
+
+
+def _fake_streaming_client(lines: list[str], *, status_code: int = 200) -> MagicMock:
+    """Return an httpx.AsyncClient stand-in whose .stream() yields a FakeSSEResponse."""
+    response = _FakeSSEResponse(lines, status_code=status_code)
+
+    @asynccontextmanager
+    async def _ctx(*_args, **_kwargs):
+        yield response
+
+    http = MagicMock()
+    http.stream = lambda *a, **kw: _ctx(*a, **kw)
+    return http
+
+
+class TestLifecycle:
+    @pytest.mark.asyncio
+    async def test_start_returns_early_when_phone_missing(self):
+        """start() with an empty phone number must not enter the HTTP loop."""
+        ch = _make_channel(phone_number="")
+        await ch.start()
+        assert ch._running is False
+        assert ch._http is None
+        assert ch._sse_task is None
+
+
+class TestSSEReceiveLoop:
+    @pytest.mark.asyncio
+    async def test_dispatches_valid_envelope(self):
+        ch = _make_channel()
+        ch._running = True
+
+        captured: list[dict] = []
+
+        async def capture(params):
+            captured.append(params)
+
+        ch._handle_receive_notification = capture  # type: ignore[method-assign]
+        ch._http = _fake_streaming_client(
+            ['data: {"envelope":{"sourceNumber":"+19995550001"}}', ""]
+        )
+
+        # Loop ends when lines exhaust; the surrounding _start_http_mode would
+        # treat that as a disconnect, but the loop itself raises ConnectionError
+        # when the stream closes while still running.
+        with pytest.raises(ConnectionError):
+            await ch._sse_receive_loop()
+        assert captured == [{"envelope": {"sourceNumber": "+19995550001"}}]
+
+    @pytest.mark.asyncio
+    async def test_handles_invalid_json_frame(self):
+        """An unparseable SSE frame is logged and skipped without crashing."""
+        ch = _make_channel()
+        ch._running = True
+
+        captured: list[dict] = []
+
+        async def capture(params):
+            captured.append(params)
+
+        ch._handle_receive_notification = capture  # type: ignore[method-assign]
+        ch._http = _fake_streaming_client(
+            [
+                "data: this-is-not-json",
+                "",  # event boundary triggers parse attempt
+                'data: {"envelope":{"sourceNumber":"+1"}}',
+                "",
+            ]
+        )
+
+        with pytest.raises(ConnectionError):
+            await ch._sse_receive_loop()
+        # Bad frame skipped; good frame still dispatched.
+        assert captured == [{"envelope": {"sourceNumber": "+1"}}]
+
+    @pytest.mark.asyncio
+    async def test_non_200_status_raises(self):
+        ch = _make_channel()
+        ch._running = True
+        ch._http = _fake_streaming_client([], status_code=503)
+        with pytest.raises(ConnectionError, match="status 503"):
+            await ch._sse_receive_loop()
+
+    @pytest.mark.asyncio
+    async def test_no_http_client_raises(self):
+        ch = _make_channel()
+        ch._http = None
+        with pytest.raises(RuntimeError, match="HTTP client not initialized"):
+            await ch._sse_receive_loop()
+
+
 # ---------------------------------------------------------------------------
 # Command handling
 # ---------------------------------------------------------------------------