[mlir-lsp] Initialize Reply::method member (#89857)

When debug level logging is enabled (by adding a call to
`Logger::setLogLevel(Logger::Level::Debug)`), the
`TransportInputTest.RequestWithInvalidParams` unit test logs:

```
[18:35:00.565] --> reply:(92)
```

The format string for this log statement is `"--> reply:{0}({1})"`,
where `{0}` is the original request's method name (that is, the method
name of the request being replied to), and `{1}` is the request ID.
However, because the `Reply` class never initializes its `method`
member, `{0}` is always empty. Initializing it results in the (nicer)
log error below:

```
I[18:35:00.565] --> reply:invalid-params-request(92)
```

Because this is only ever logged for now, its not possible to add a test
case for this. Future patches will rely on `method` being initialized,
however, and will add test cases for this path.