Fix URI parser: IPv6 structural validation and pct-encoded case sensitivity

[AcEKaycgR]

Fixes #2353

What changed

IPv6 false positives (6 cases)

parse_ipv6() previously accepted any sequence of hex digits, colons, and dots inside brackets without validating RFC 3986 §3.2.2 / RFC 4291 structural rules. After the character-level scan, the extracted address is now passed to is_ipv6() which already enforces all the rules correctly:

• http://[] - empty brackets
• http://[2001:db8::00000] - h16 group exceeds 4 hex digits
• http://[2001::db8::1] - more than one ::
• http://[1:2:3:4:5:6:7] - 7 groups without ::
• http://[1:2:3:4:5:6:7:8:9] - 9 groups
• http://[::ffff:1.2.3.256] - embedded IPv4 octet out of range

Percent-encoding false negative (1 case)

validate_percent_encoded_utf8() was treating bytes like 0xAF as bare UTF-8 continuation bytes and throwing. RFC 3986 §2.1 only requires two valid HEXDIG after %; UTF-8 validity of the encoded octet is not required. The function now returns after confirming two valid hex digits.

• http://a.com/%aF - mixed-case hex rejected incorrectly

Tests

7 regression tests added to uri_parse_test.cc, one per fixed case.
All 628 existing URI unit tests continue to pass.

Its ready for review @jviotti

[augmentcode]

🤖 Augment PR Summary

Summary: This PR tightens URI parsing around IP-literals and percent-encoding to better match RFC 3986.

Changes:

• Guarded UNC path detection in URI::from_path so //-prefixed paths are treated as UNC only on Windows.
• Simplified percent-encoding validation to require only % + two HEXDIG (no UTF-8 byte-sequence validation), fixing mixed-case hex handling.
• Improved IPv6 parsing by validating bracket contents structurally via sourcemeta::core::is_ipv6() after the character-level scan.
• Adjusted path/query/fragment scanners to advance cleanly after consuming a percent-encoded triplet.
• Updated existing percent-encoding negative tests to use truly invalid hex and added regression tests for mixed-case percent-encoding and IPv6 structural false positives.

Technical Notes: IPv6 validation now leverages the existing core IP validator, while IPvFuture handling remains parser-local per RFC 3986.

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[augmentcode]

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[cubic-dev-ai]

No issues found across 3 files

[jviotti]

Ahh good catch. We developed the ip module very recently. Makes sense using here now! 💯

[jviotti]

Impeccable PR. Great job. Thanks!