[Bug] Fix NPE when recording RPC latency for local proxy requests

### Before Creating the Bug Report

- [x] I found a bug, not just asking a question, which should be created in [GitHub Discussions](https://github.com/apache/rocketmq/discussions).

- [x] I have searched the [GitHub Issues](https://github.com/apache/rocketmq/issues) and [GitHub Discussions](https://github.com/apache/rocketmq/discussions) of this repository and believe that this is not a duplicate.

- [x] I have confirmed that this bug belongs to the current repository, not other repositories of RocketMQ.

### Runtime platform environment

All

### RocketMQ version

5.x

### JDK Version

All

### Describe the Bug

In Local Proxy mode, `LocalRemotingCommand` does not initialize `processTimer` because the request does not pass through `NettyDecoder`.

This causes a `NullPointerException` in the `ChannelFutureListener` of `NettyRemotingAbstract.writeResponse` when recording the RPC latency.

### Steps to Reproduce

1. Start RocketMQ Proxy in Local mode.
2. Send a message through the gRPC endpoint.
3. Inspect `NettyRemotingAbstract.writeResponse` when the response is written.
4. Observe that `request.getProcessTimer()` is `null`.

### What Did You Expect to See?

The local request should have an initialized process timer, and the RPC latency metric should be recorded successfully.

### What Did You See Instead?

A `NullPointerException` occurs when calling `request.getProcessTimer().elapsed(...)`.

### Additional Context

_No response_

Start by tracing LocalRemotingCommand and NettyRemotingAbstract.writeResponse, then compare the request path with NettyDecoder, where processTimer is initialized. Reproduce the issue in RocketMQ Proxy Local mode through the gRPC endpoint; done means the response path no longer throws a NullPointerException and RPC latency is recorded successfully.

Written by the indexing model from the issue text.