feat(BOUN-1955): Second part of SMTP implementation

[blind-oracle]

• Implement to SMTP->IC part: IcSmtpDeliveryAgent
• Hide SMTP stuff under a feature flag
• Performance improvements in SMTP library (use ArrayString, pre-render EHLO/HELO etc)
• Other changes here and there

[Copilot]

Pull request overview

This PR completes the “SMTP → IC” direction by introducing an IC-backed delivery/recipient-resolution implementation and gating the SMTP subsystem behind a new smtp feature, alongside performance-focused changes in the SMTP inbound server.

Changes:

• Added IC SMTP request execution + IcSmtpDeliveryAgent to validate recipients and deliver parsed MIME messages to IC canisters.
• Optimized SMTP inbound path (pre-rendered HELO/EHLO responses, reduced allocations for replies/errors).
• Introduced smtp feature flag and updated dependencies/exports accordingly.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
ic-bn-lib/tools/smtp_server.rs	Removes the standalone SMTP server tool binary source.
ic-bn-lib/src/smtp/server.rs	Adds accept() failure throttling and cleans imports.
ic-bn-lib/src/smtp/mod.rs	Renames/extends core SMTP types (e.g., EmailMessage, richer resolve errors, from passed to resolver).
ic-bn-lib/src/smtp/inbound/session.rs	Allocation reductions in replies/errors; adjusts state machine responses.
ic-bn-lib/src/smtp/inbound/rcpt_to.rs	Passes sender to resolver; richer error mapping; avoids heap formatting.
ic-bn-lib/src/smtp/inbound/mod.rs	Precomputes HELO/EHLO responses; adds SMTP caps; updates/extends tests.
ic-bn-lib/src/smtp/inbound/mail_from.rs	Adds sender-domain validation and allocation reduction for SPF error messages.
ic-bn-lib/src/smtp/inbound/ehlo.rs	Switches EHLO/HELO output to pre-rendered buffers.
ic-bn-lib/src/smtp/ic/mod.rs	Adds IC SMTP request executor trait/impl and MIME→candid message parsing.
ic-bn-lib/src/smtp/ic/delivery_agent.rs	Implements IC-backed delivery + recipient validation, including MX canister lookup + caching.
ic-bn-lib/src/smtp/ic/candid.rs	Updates candid model (notably Envelope.to becomes a list) and derives equality.
ic-bn-lib/src/smtp/address.rs	Adds conversions to candid Address and parsing helper; adds PartialEq<&str>.
ic-bn-lib/src/lib.rs	Gates smtp module behind feature; adds email! macro.
ic-bn-lib/src/custom_domains/mod.rs	Introduces LooksUpCustomDomain abstraction for SMTP IC integration.
ic-bn-lib/Cargo.toml	Adds smtp feature; makes SMTP deps optional; removes smtp-server bin entry; adds dev-dep for tests.
Cargo.toml	Adds workspace deps for arrayvec and mail-send.

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[blind-oracle]

@frankdavid thanks for the review! 👍 I'll merge this as-is now and will address the comments in the next PR for simplicity.