Skip handling when handler is closed to prevent processing on canceled calls

[HyprUsr]

Summary

This PR fixes a critical server-side crash occurring when a gRPC client disconnects immediately after initiating a request. This issue typically results in an unhandled Bad state: Cannot add event after closing error, which brings down the entire server process.

Related Issues

Fixes #67 and #558 . This behavior has been reported previously but remained unresolved until now.

The Problem

When a client terminates abruptly (e.g., a process crash or network drop) immediately after sending a request, the server-side stream may transition to a closed state while the handler is still attempting to send data or events back. Attempting to interact with the closed stream without a state check throws a fatal "Bad state" exception.

As a matter of principle, a client-side disconnection should never be able to compromise the stability of the server process.

The Solution

The fix introduces a safety check on the isCanceled flag prior to any interaction with the client stream. This ensures that if the connection is already dead, the server gracefully handles the state rather than attempting to write to a closed sink.

How to Reproduce

1. Start the gRPC server.
2. Run the following Go script. It opens a stream, sends a single message, and then calls os.Exit(0) to simulate an abrupt termination without a clean shutdown.
3. Observe the server logs for the Bad state: Cannot add event after closing crash.

package main

import (
	"context"
	"log"
	"os"
	"time"

	pb "your/module/path/protos" 
	"google.golang.org/grpc"
)

func main() {
	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
	defer cancel()

	conn, err := grpc.DialContext(ctx, "localhost:50051",
		grpc.WithInsecure(),
		grpc.WithBlock(),
		grpc.WithDefaultCallOptions(grpc.WaitForReady(true)),
	)
	if err != nil {
		log.Fatalf("dial: %v", err)
	}

	client := pb.NewMyServiceClient(conn)
	stream, err := client.Chat(ctx) 
	if err != nil {
		log.Fatalf("open stream: %v", err)
	}

	if err := stream.Send(&pb.ChatRequest{Message: "hi"}); err != nil {
		log.Fatalf("send: %v", err)
	}

	// Terminate immediately to mimic abrupt client death.
	os.Exit(0)
}