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]

I noticed it's possible to do the following:

• User A runs the format code command
• User B runs the format code command
• User A deletes the code they formatted
• User A changes the language from their ephemeral message

In this case, the language of the code block of user B is changed. Could you prevent this in one of the following ways?

• In LinkedMessages.resolveAfter, check that the last message contains the delete button with the delete button with the correct requestor (= current user) and that no other messages have a delete button
• Change the ephemeral message to be sent after the formatted codeblocks, use resolveBefore and add an additional argument containing the message ID of the first code block message to the ID here. If this doesn't match, it is a failure.

I think the first approach is easier to do so I recommend that one.