test(ipv6): assert a compressed address at the maximum colon count is valid

[vtushar06]

The third IPv6address alternative in RFC 3986 Section 3.2.2 is

/ [               h16 ] "::" 4( h16 ":" ) ls32

With the optional prefix absent and ls32 = h16 ":" h16, that is ::1:2:3:4:5:6. The alternative has no valid test in the current file.

@exodus/schemasafe 1.3.0 rejects it, while passing all 40 tests the file has today. Its ipv6 checker is a hand-written character scan rather than a regex, and it ends with (src/formats.js):

const spaces = s1 > 0 ? 6 : 7
if (!short) return s0 === spaces && start && hex > 0
return (start || hex > 0) && s0 < spaces

s0 is the colon count, s1 the dot count, and short is true when a :: was seen. spaces is the colon count of an uncompressed address: seven for eight groups, six for six groups plus a dotted quad. The compressed branch on the last line reuses that same number, but an elision adds a colon to the string. The real maxima are 8 and 7, so every compressed address sitting at the top of its colon range is rejected:

input	colons	schemasafe
::1:2:3:4:5:6	7	rejects
1:2:3:4:5:6:7::	8	rejects
1::2:3:4:5:6:7	7	rejects
1:2:3:4:5::1.2.3.4	6	rejects
1:2:3:4:5::	6	accepts
::ffff:192.168.0.1	3	accepts

The file's nine compressed valid cases - ::1, ::abef, ::, ::42:ff:1, d6::, 1:d6::42, 1::d6:192.168.0.1, 1:2::192.168.0.1, ::ffff:192.168.0.1 - all sit at six colons or fewer, which is why none of them reaches this.

I picked the seven-colon pure-hex form as the representative because it also fills the missing third alternative. 1:2:3:4:5:6:7:: fails the same way but is a shape the file already covers through d6::.

Changes

One test, added to the six files where format: ipv6 exists:

• ::1:2:3:4:5:6 - valid

Ecosystem Impact

• @exodus/schemasafe 1.3.0 (JavaScript): FAILS - rejects. 1.3.0 is the current release; there is no newer one to upgrade to.
• ajv-formats 3.0.1 (full and fast): PASSES - accepts.
• python-jsonschema 4.25.1: PASSES - accepts.
• json_schemer 2.2.1, fastjsonschema 2.21.2, jsonschema-rs 0.34.0, @cfworker/json-schema 4.1.1, santhosh-tekuri/jsonschema v6.0.2, xeipuuv/gojsonschema 1.2.0, networknt/json-schema-validator 1.5.6, opis/json-schema, justinrainbow/json-schema, boon 0.6.1, sourcemeta/core: PASSES - all accept.

RFC References

• RFC 3986 Section 3.2.2 - the IPv6address ABNF, third alternative.
• RFC 4291 Section 2.2 - form 2, the :: elision.

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