fix(imap): drain server greeting before login

[tymrtn]

Summary

• Drains the initial IMAP server greeting before issuing LOGIN in normal IMAP connections.
• Applies the same greeting drain to IDLE/watch connections.
• Adds transport tests for successful greeting drain and EOF-before-greeting error reporting.

User-facing surface

• Public commands are unchanged.
• Improves reliability for existing envelope ... IMAP workflows, including inbox/search/backup/watch paths that need a fresh IMAP connection.

Internal verification

• cargo fmt --check
• cargo test -p envelope-email-transport imap -- --nocapture
• cargo test -p envelope-email-transport idle -- --nocapture
• cargo test -p envelope-email-transport -- --nocapture

Safety

• No live mailbox export, restore, migration, or message mutation was run.
• No credentials inspected.