fix: reject invalid dotted-decimal in hostname_rfc1123 validation

[deepakganesh78]

Fixes #1561

Problem

hostname_rfc1123 accepted strings like 277.168.0.1 as valid hostnames. Per RFC 1123 §2.1, a valid hostname can never have the dotted-decimal form #.#.#.# since the highest-level component label must be alphabetic.

Root Cause

The hostnameRegexRFC1123 regex allows all-digit labels, so a four-part all-numeric string like 277.168.0.1 passes the regex even though it is neither a valid IPv4 address nor a legitimate hostname.

Fix

After the regex matches, a new looksLikeIPv4 helper checks whether the string has exactly four dot-separated all-numeric parts (the dotted-decimal form). Only in that case is net.ParseIP called; invalid addresses are rejected. The guard is deliberately narrow:

Input	looksLikeIPv4	Result	Rationale
192.168.0.1	true	accepted	Valid IPv4
0.0.0.0	true	accepted	Valid IPv4
255.255.255.255	true	accepted	Valid IPv4
277.168.0.1	true	rejected	Octet 277 out of range
999.999.999.999	true	rejected	All octets out of range
1.2.3.256	true	rejected	Octet 256 out of range
3com.com	false	accepted	Contains letters, not dotted-decimal
1and1.com	false	accepted	Contains letters
7-eleven.com	false	accepted	Contains letters and hyphens
1234	false	accepted	Single all-numeric label (no dots, not 4-part) — valid per RFC 1123
12345	false	accepted	Same as above
1.2.3	false	accepted	3 numeric parts, not dotted-decimal form
1.2.3.4.5	false	accepted	5 numeric parts, not dotted-decimal form

This avoids calling net.ParseIP for the vast majority of inputs and does not affect fqdn, hostname_port, or other validators (fqdn already requires an alphabetic TLD; hostname_port calls hostnameRegexRFC1123 on the host part only after stripping the port).

Validation

• go test ./... — all 24 packages pass
• go vet ./... — clean
• gofmt -l — clean on touched files

Compatibility

No breaking change. Valid IPv4 addresses continue to be accepted. Only invalid four-part dotted-decimal strings that were incorrectly accepted are now rejected.