fix(spammer): return errors for invalid inputs

[kriss39]

Summary

• return user-facing errors instead of panicking for invalid direct WS targets
• make rate limiter construction fallible for oversized TPS / burst values
• add regression coverage for invalid WS target parsing and oversized TPS

Testing

• git diff --check
• not run: cargo fmt --check / cargo test -p spammer (cargo is not installed in this environment)

[ZhiyuCircle]

Hi @kriss39,

Thank you for your interest in contributing to Arc Node, and apologies for the delay in getting back to this PR.

We're closing out the pull request backlog that predates our current contribution policy. This PR is being closed because it does not reference a GitHub issue. All PRs must reference an existing issue using the format Closes: #XXX, and the author must be assigned to that issue before the PR is opened.

This is not a judgement on the change itself. If you'd still like to land it:

1. Open an issue describing the problem, or find the existing one
2. Comment on the issue requesting assignment, and wait for maintainer approval
3. Open a fresh PR once you have been assigned

Please see CONTRIBUTING.md for details. Thanks again for taking the time to contribute.