Remove unused username binding

[Errordog2]

Closes #32.

Summary

• Remove the unused _username parameter from �alidate_input().
• Remove the dead let _ = _username; binding.
• Update the two call sites to pass only the fields that are actually validated.

Validation

• Static search confirmed let _ = _username, _username:, and old �alidate_input(&env, Some/None, ...) call sites are gone.
• git diff --check passed.
• Sensitive scan of src/lib.rs passed.

Note: cargo fmt / cargo fmt --check were attempted locally but blocked by rustup/toolchain startup hanging on this Windows host; no functional code paths changed.