fix: NewJetProducer ctx is nil causing crash

[4566704]

NewJetProducer 创建 JetProducer struct时没有赋值 ctx，因为JetProducer的一些方法要用到ctx，ctx为nil引发panic:

Greptile Summary

Fixed a critical nil context issue in the NATS JetStream producer implementation that was causing panics during stream operations.

• Added missing context parameter to natsq/producer.go:NewJetProducer() constructor and properly initialized it in the JetProducer struct
• Updated example/natsq/publisher/publisher.go to pass context.Background() to NewJetProducer calls
• Error handling in example/natsq/publisher/publisher.go should be improved for CreateOrUpdateStream operations
• Resource cleanup could be improved by using defer for Close() calls in publisher example

_{💡 (1/5) You can manually trigger the bot by mentioning @greptileai in a comment!}

[greptile-apps]

_{2 file(s) reviewed, 1 comment(s)
Edit PR Review Bot Settings | Greptile}

[greptile-apps]

logic: Error handling is ignored. Could lead to nil pointer dereference if producer creation fails.

Suggested change

	j, _ := natsq.NewJetProducer(&c, context.Background())
	j, err := natsq.NewJetProducer(&c, context.Background())
	if err != nil {
	log.Fatalf("Error creating jet producer: %v", err)
	}

[greptile-apps]

LGTM

_{2 file(s) reviewed, no comment(s)
Edit PR Review Bot Settings | Greptile}

[4566704]

BUG说明:
1.修复前 example/natsq/publisher/publisher.go 是没办法正常运行的，会引发panic
2.JetProducer struct 定义了 ctx context.Context，但没有赋值，JetProducer的一些方法使用了它。

修复说明:
1.NewJetProducer方法追加了ctx 参数。
2.尽可能少改动代码和不新增方法。