Use FormValidation objects

[hellopablo]

At the moment there are some rules baked into nails/common. these should be provided as FM Validation objects as per nails/common#157.

[hellopablo]

Triage

1. Still applicable? No — appears done. The FormValidation service is now used pervasively across the module (src/Settings/General.php, src/Admin/Controller/Import.php, src/Api/Controller/MediaManagerV2.php, etc.), rather than validation rules being baked in as raw constants.
2. Blast radius — N/A.
3. Effort — N/A.
4. Value — N/A.

Closing as completed — FormValidation objects are the current, established pattern in this codebase.

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