Fix Great Vault Escape handling when opened through macros or other addons

[andybergon]

What does this PR do?

Make Escape close the Great Vault consistently when it is opened through another addon or a macro, as it already does through EllesmereUI's minimap and Data Bars shortcuts.

Currently, those EUI shortcuts register the Vault with the shared Escape handler when clicked. Opening it through another route first misses that registration. Register it once when the Vault UI is available instead, and remove the repeated registrations from the shortcuts.

How was it tested?

• Lua 5.1 compilation passed for all three changed files; git diff --check passed.
• Mocked behavior checks passed for late loading, an already-loaded hidden or visible Vault, repeated opening and closing, listener cleanup, and Escape with another EUI window open.
• Installed locally with backup and hash verification. In-game testing is not yet confirmed.

Checklist

• N/A: No new settings; this fixes inconsistent existing Escape behavior.
• N/A: No separate enable/disable setting. The load listener unregisters once the Vault is registered.
• Cheap while enabled: event-driven, no polling or timers.
• No writes onto Blizzard-owned frames; uses the existing Escape handler's HookScript integration.
• Tested in-game on live. No version gates or pre-Midnight APIs added.