feat(NODE-1955): SMTP: More observability, add notifications handler etc

[blind-oracle]

• Adds handler for message delivery / session finish notifications - needed for further integration with Vector to emit events (ReceivesNotifications trait)
• Track last error during session (ProtocolError)
• Move tests to tests.rs
• More detailed logging here and there

[Copilot]

Pull request overview

This PR extends the SMTP inbound stack with richer observability by introducing notification hooks, tracking protocol/message errors in session metadata, and updating the delivery agent interface to receive session context.

Changes:

• Add ReceivesNotifications, MessageError, and ProtocolError to emit message/session/protocol events and track last protocol error.
• Thread SessionMeta through delivery and notification paths; update SMTP session logic to record/report protocol errors.
• Move inbound SMTP tests into a dedicated tests.rs module and expand tests to cover notifications.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
ic-bn-lib/src/smtp/mod.rs	Adds notification trait + structured error enums; changes delivery trait to accept SessionMeta.
ic-bn-lib/src/smtp/inbound/mod.rs	Introduces SessionMeta, tracks message_id/last_error, and adds notification plumbing (set_error).
ic-bn-lib/src/smtp/inbound/session.rs	Records protocol errors, emits message notifications (success/failure), and uses stable message_id per transaction.
ic-bn-lib/src/smtp/inbound/manager.rs	Adds session-finish notifications and refactors TLS handling/handshake error reporting.
ic-bn-lib/src/smtp/inbound/ehlo.rs	Emits ProtocolError for EHLO validation failures.
ic-bn-lib/src/smtp/inbound/mail_from.rs	Emits ProtocolError for MAIL FROM sequencing/validation/SPF/reverse-IP failures.
ic-bn-lib/src/smtp/inbound/rcpt_to.rs	Emits ProtocolError for RCPT TO sequencing/validation/resolution failures.
ic-bn-lib/src/smtp/inbound/tests.rs	New extracted inbound SMTP test module; adds notification handler coverage.
ic-bn-lib/src/smtp/ic/delivery_agent.rs	Updates IC delivery agent to accept SessionMeta and adjusts logging/tests.
ic-bn-lib/src/lib.rs	Re-exports rcgen.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.