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)