Reply with canvas error when canvas callback fails synchronously

Address Cloud Code Review feedback: the canvas.open / canvas.close /
canvas.action.invoke handlers caught synchronous failures (e.g. malformed
params from treeToValue, or any logic before the CompletableFuture chain is
attached) but only logged them, leaving the caller without a response. Send
a structured canvas error reply in the catch block so the provider callback
always completes instead of hanging.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: jmoseley

java/src/main/java/com/github/copilot/RpcHandlerDispatcher.java

@@ -498,6 +498,7 @@ private void handleCanvasOpen(JsonRpcClient rpc, String requestId, JsonNode para
                 });
             } catch (Exception e) {
                 LOG.log(Level.SEVERE, "Error handling canvas open request", e);
+                sendCanvasError(rpc, requestIdLong, "canvas.open", e);
             }
         });
     }
@@ -527,6 +528,7 @@ private void handleCanvasClose(JsonRpcClient rpc, String requestId, JsonNode par
                 });
             } catch (Exception e) {
                 LOG.log(Level.SEVERE, "Error handling canvas close request", e);
+                sendCanvasError(rpc, requestIdLong, "canvas.close", e);
             }
         });
     }
@@ -556,6 +558,7 @@ private void handleCanvasActionInvoke(JsonRpcClient rpc, String requestId, JsonN
                 });
             } catch (Exception e) {
                 LOG.log(Level.SEVERE, "Error handling canvas action invoke request", e);
+                sendCanvasError(rpc, requestIdLong, "canvas.action.invoke", e);
             }
         });
     }