feat: add comprehensive unit and integration test suite

With the help of Copilot.

Unit tests (103 tests, 410 assertions):

• WafIpUtilsTest: waf_sanitize_ip, waf_parse_ip_list, waf_ip_in_list
• WafCidrTest: waf_ti_ip_in_cidr, waf_ti_ip6_in_cidr, waf_ti_is_local_ip, waf_ti_local_cidrs, waf_ti_local_cidrs_v6
• WafPrefixTreeTest: waf_ti_build_prefix_tree, waf_ti_lookup_ip (exact, /8, /16, /24, non-octet-aligned CIDRs, ancestor-override, edge cases)
• WafBadgeFiltersTest: waf_type_badge (all event types + XSS escape), waf_reason_class
• WafPatternDetectionTest: waf_malicious_payload_patterns (data-provider with 12 attack vectors + 5 safe payloads), waf_suspicious_ua_patterns (11 scanner UAs), waf_ti_parse_lines
• WafCachePathTest: _waf_cache_path (subdirectory layout, extension)
• WafFlattenTest: waf_flatten_to_strings (nested arrays, scalar casts)

Integration tests (11 tests, 24 assertions):

• WafCacheIntegrationTest: file-based cache round-trip (scalar + array), miss, delete, TTL expiry, no-expiry, waf_cache_clean (expired vs fresh), overwrite