fix: validate malformed IPv6 zone identifiers

[Tony133]

Proposal:

Fix malformed IPv6 zone identifier handling in bracketed IP literals.

When parsing IPv6 zone identifiers, malformed values such as:

• fe80::1%25
• fe80::1%25eth 0
• fe80::1%25eth%ZZ
• fe80::1%25K

must be rejected rather than silently rewritten.

This is particularly relevant on Node.js 16, where the native URL parser can truncate these values instead of reporting an invalid host.

Changes:

• Validate IPv6 zone identifiers explicitly in normalizeIPv6().
• Reject empty, whitespace-containing and malformed percent-encoded zone identifiers.
• Preserve the original malformed host instead of truncating or rewriting it.
• Add regression tests for valid and malformed IPv6 zone identifiers.

Note:

• Tested with Node.js 16.20.2
• Fix test in CI ( see screenshot ) or https://github.com/fastify/fast-uri/actions/runs/32536858344/job/97013696061#step:5:244

[mcollina]

@Tony133 oops, this now conflicts. PTAL.

[Tony133]

@mcollina conflicts resolved ✅ 👀