fix: replace bool comparison with negation to fix clippy lint error

[JiayuuWang]

Summary

Fixes the clippy lint error reported in #939.

cargo clippy --workspace --all-targets -- -D warnings fails with:

error: equality checks against false can be replaced by a negation
   --> crates/openfang-runtime/src/web_fetch.rs:517:17
    |
517 |         assert!(is_host_allowed("other.net", &allow) == false);
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `!is_host_allowed("other.net", &allow)`
    |
    = note: `-D clippy::bool-comparison` implied by `-D warnings`

Change

Replace assert!(expr == false) with the idiomatic assert!(!expr) in the test_ssrf_allowlist_wildcard_domain test.

Closes #939