Fix queued mail attachments and stale automatic subjects

[aleksandr-kotlyar]

Summary

Fix two issues affecting queued bulk mail sent via Alt+Click:

• Previous attachments could remain in the send slots as ?, requiring repeated clicks on Send.
• The automatically generated subject could remain unchanged when sending a different item batch to the same recipient.

Changes

• Wait for all mail attachment slots to clear after MAIL_SEND_SUCCESS before processing the next queued mail.
• Listen for MAIL_SEND_INFO_UPDATE instead of relying only on a fixed delay.
• Check every attachment slot rather than only the first one.
• Add a bounded retry mechanism with a helpful error message if the slots never clear.
• Generate the automatic subject for each outgoing batch.
• Use the total item count when a batch contains multiple stacks of the same item.
• Preserve subjects and recipients entered manually by the player.
• Pin the embedded LibMagicUtil external to the localization-safe commit proposed in LibMagicUtil PR #22. This prevents parent addon localization data from being written into library locale files.

Testing

• Tested in-game on the TBC Anniversary client (2026-09-20).
• Lua 5.1 syntax validation passes for BulkMail and all locale files.
• The package metadata parses successfully and resolves the pinned LibMagicUtil commit.