test: cover branch where reconnection exhausts max attempts

Add test_sse_error_when_reconnection_exhausted to exercise the
_handle_sse_response path where SSE events are received (setting
last_event_id) but reconnection fails, ensuring the JSONRPCError
is sent to unblock the waiting request.

Author: heyhayes

tests/shared/test_streamable_http.py

@@ -2275,6 +2275,33 @@ async def test_sse_read_timeout_propagates_error(basic_server: None, basic_serve
                         await session.read_resource("slow://test")
 
 
+@pytest.mark.anyio
+async def test_sse_error_when_reconnection_exhausted(
+    event_server: tuple[SimpleEventStore, str],
+    monkeypatch: pytest.MonkeyPatch,
+):
+    """When SSE stream closes after events and reconnection fails, MCPError is raised."""
+    _, server_url = event_server
+
+    async def _always_fail_reconnection(
+        self: Any, ctx: Any, last_event_id: Any, retry_interval_ms: Any = None, attempt: int = 0
+    ) -> bool:
+        return False
+
+    monkeypatch.setattr(StreamableHTTPTransport, "_handle_reconnection", _always_fail_reconnection)
+
+    async with streamable_http_client(f"{server_url}/mcp") as (read_stream, write_stream):
+        async with ClientSession(read_stream, write_stream) as session:
+            await session.initialize()
+
+            # tool_with_stream_close sends a priming event (setting last_event_id),
+            # then closes the SSE stream. With reconnection patched to fail,
+            # _handle_sse_response falls through to send the error.
+            with pytest.raises(MCPError):
+                with anyio.fail_after(10):
+                    await session.call_tool("tool_with_stream_close", {})
+
+
 @pytest.mark.anyio
 async def test_handle_reconnection_returns_false_on_max_attempts():
     """_handle_reconnection returns False when max attempts exceeded."""