fix(imp): prevent search and subscribe PHP warnings

[TDannhauer]

Summary

• Normalize IMP_Search_Query mailbox lists to IMP_Mailbox objects before deduplication
• Ensure IMP_Mailbox::subscribe() always sets a success notification when the parent mailbox action succeeded

Motivation

array_unique(..., SORT_REGULAR) could keep mailbox path strings instead of IMP_Mailbox objects when the same mailbox appeared as both, causing Attempt to read property "container" on string in search query building. With subfolders: true, ignored subfolder subscribe errors left $notify undefined even though the parent mailbox subscribe/unsubscribe had already succeeded.

Changes

• Build the cached mboxes list by normalizing through IMP_Mailbox::get() and deduplicating by mailbox name
• Fall back to the single-mailbox success message when no subfolder action succeeded

Test plan

• Run a mailbox search spanning folders with subfolder expansion
• Subscribe/unsubscribe a mailbox with the subfolders option when some subfolders fail on the server