Fix IPv4 URL safety normalization

[adulau]

Motivation

• Guard access to ipv4_mapped when normalizing parsed host IPs so plain IPv4 literals or hostnames resolving to IPv4 addresses do not raise AttributeError and crash URL safety checks.

Description

• Replace ip.ipv4_mapped or ip with getattr(ip, "ipv4_mapped", None) or ip in misp_modules/modules/expansion/html_to_markdown.py to safely handle IPv4Address and IPv6Address values and preserve IPv4-mapped IPv6 behavior, and add two regression tests (test_allows_public_ipv4_literal and test_allows_hostnames_resolving_to_public_ipv4_addresses) to tests/test_html_to_markdown.py.

Testing

• Ran python -m pytest tests/test_html_to_markdown.py -q and all tests passed (6 passed, 5 subtests passed).

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Codex Task