gh-63121: Refresh imaplib capabilities on state changes (GH-152752)

gh-63121: Refresh imaplib capabilities on state changes (GH-152752)

imaplib fetched the server capabilities only once, at connection time.
They are now also refreshed after a successful LOGIN or AUTHENTICATE,
from the CAPABILITY response the server sent or, if it sent none, by
querying it.  This lets methods such as enable() see capabilities added
after login, for example ENABLE on Gmail (gh-103451).

Capabilities advertised in the server greeting are now used too, saving
a redundant CAPABILITY command.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>