[6.x] Fix clipboard pasting of validation rules

[joshuablum]

Pasting a validation rule such as mimes:jpg,jpeg,png,webp into a validation rules field (general blueprint or asset container config) split it on every comma resulting in four separate and wrong rules instead of one. The reason is that Combobox's paste handler always splits the pasted text on commas when taggable is true. Typing the rule by hand works fine.

Behavior now:

• Pasting mimes:jpg,jpeg,png,webp is one rule.
• Pasting required|mimes:jpg,jpeg,png,webp is two rules.
• Fixed in both places (general blueprints and the asset container config).
• Regular tags fields keep splitting on commas as before.

Closes #14750.