fix(security): block private/loopback/metadata IPs in link-understanding URL detection (#15604)
* fix(security): block private/loopback/metadata IPs in link-understanding URL detection
isAllowedUrl() only blocked 127.0.0.1, leaving localhost, ::1, 0.0.0.0,
private RFC1918 ranges, link-local (169.254.x.x including cloud metadata),
and CGNAT (100.64.0.0/10) accessible for SSRF via link-understanding.
Add comprehensive hostname/IP blocking consistent with the SSRF guard
already used by media/fetch.ts.
* fix(security): harden link-understanding SSRF host checks
* fix: note link-understanding SSRF hardening in changelog (#15604) (thanks @AI-Reviewer-QS)
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Co-authored-by: Yi LIU <yi@quantstamp.com>
Co-authored-by: Peter Steinberger <steipete@gmail.com>
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