enable remote verification by default in RExecClient

[dxbjavid]

RExecClient documents that verification of the remote host providing a separate error stream is enabled by default, and the sibling FTPClient initialises the same flag to true, but here the field is left uninitialised so it defaults to false. That means the anti-hijacking check in createErrorStream is skipped unless a caller turns it on by hand, so an attacker who is monitoring the rexec or rcmd negotiation can race a connection to the advertised error-stream port and have it accepted as the remote process's stderr. This initialises the flag to true so the documented default is honoured; RCommandClient and RLoginClient inherit the field, and the added tests fail on master and pass with the change.

• Read the contribution guidelines for this project.
• Read the ASF Generative Tooling Guidance if you use Artificial Intelligence (AI).
• I used AI to create any part of, or all of, this pull request. Anthropic's Claude (via Claude Code) helped locate the issue, draft the one-line change and the unit tests, and write this description; I reviewed the change and verified the build and tests.
• Run a successful build using the default Maven goal with mvn; that's mvn on the command line by itself.
• Write unit tests that match behavioral changes, where the tests fail if the changes to the runtime are not applied. This may not always be possible, but it is a best practice.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Each commit in the pull request should have a meaningful subject line and body. Note that a maintainer may squash commits during the merge process.