feat: delete-all button and language dropdown for multi-message code blocks

[Neil-Tomar]

What

Multi-message formatted code blocks now have two interactions, backed by a small message-linking helper:

• Delete-all button — removes every message of the block in one click
• Language dropdown (ephemeral) — re-colors every chunk to a chosen language

How

• LinkedMessages resolves the block from the component's message (getHistoryBefore/After), touching only the bot's own messages.
• FormatCodeInteractionHandler handles the delete button and the language dropdown.
• Permission: requester / staff / owner (same rule as the existing delete button).

Open questions

• The language dropdown is ephemeral per your suggestion, so it's a one-time picker that's gone on reload — happy to make it a permanent public control instead.
• Rare edge case: two simultaneous formats could interleave bot messages; the resolver is best-effort there.

[danthe1st]

Two very minor things. If you fixed those, I'll start testing everything.

[danthe1st]

LGTM