Function parseManagerDomain

Parse a MANAGERDOMAIN directive out of an ads.txt body. Returns the lowercased host token (last-wins on duplicates) or undefined when no eligible directive is present.

Eligibility rules (per adcp-client#1717 / adcp#4175 + #4173 resolution):

• Directive form MANAGERDOMAIN=<host> only. Case-insensitive on the key; the value preserves no case. Comment-only lines like # managerdomain=... are rejected.
• Value must be a host token (no scheme, no path, no whitespace).
• Trailing inline comment containing noagents (case-insensitive) opts that entry out of fallback discovery.
• Duplicate eligible entries: the last one wins.

Exported for direct unit testing.