[cmd/mdatagen] Implement array validation in generated Go config code

[jkoronaAtCisco]

Part of #14809

Implement support for the JSON Schema array validation keywords in Go code generated by mdatagen.

Scope

• minItems: Array must contain at least the specified number of items.
• maxItems: Array must contain at most the specified number of items.
• uniqueItems: All items in the array must be unique.
• contains: At least one item in the array must match the specified schema.

Acceptance Criteria

• Generated Validate() methods enforce all four array constraints.
• Validation errors clearly indicate which constraint was violated.
• Unit tests cover edge cases (empty arrays, boundary sizes, duplicate detection, contains matching).

[github-actions]

Pinging code owners:

• cmd/mdatagen: @dmitryax

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