fix: validate color-code markers against guild regex

[darbyjack]

Summary

Fix guild name and prefix validation when guild.requirements.include-color-codes is disabled.

The setting continues to control whether valid color-code sequences count toward the configured length, but the raw color-code marker must still be permitted by the configured regex.

Root cause

The previous validation translated and stripped color codes before applying the regex. As a result, an input such as &aGuild became Guild before validation, allowing & to bypass a regex that intentionally excluded it.

Changes

• add a shared GuildInputValidator for guild names and prefixes
• strip valid color-code sequences only for visible-length validation when configured
• separately verify that alternate (&) and section-sign (§) color-code markers are accepted by the configured regex
• use the shared validation for guild creation, renaming, and prefix changes
• preserve the existing behavior when color codes are configured to count toward length

Local validation

Compiled the exact validator implementation locally with lightweight stubs and ran a standalone assertion harness covering:

• plain names
• forbidden and allowed & color codes
• color codes counted versus excluded from length
• multiple formatting codes
• literal ampersands
• prefix validation
• section-sign color codes
• invalid visible characters

Fixes #266