Add SMTP attachment support

[diesdasjunge]

Summary

Adds attachment support to the built-in SMTP adapter using the existing EmailAttachment API and shared attachmentToBytes() helper.

SMTP now accepts attachments, builds dependency-free MIME payloads, and becomes a valid fallback route for attachment-bearing messages while continuing to reject unsupported tags and metadata.

What Changed

• Enables attachments in SMTP field support.
• Builds the full MIME payload before opening an SMTP socket.
• Emits multipart/mixed when attachments are present.
• Nests multipart/alternative for text + HTML bodies.
• Supports raw string, Base64 string, byte, and file-backed attachments.
• Adds conservative validation for attachment filenames, content types, content IDs, and dispositions.
• Documents SMTP attachment support and the timeoutMs consideration for large attachments.
• Adds a minor changeset and updates the package changelog.

Verification

• bun test packages/email-sdk/src/smtp.test.ts
• bun test packages/email-sdk/src/cli.test.ts
• bun test packages/email-sdk
• bun run check-types
• bun run release:ci

Notes

• SMTP still rejects tags and metadata.
• Inline attachments emit Content-ID, but richer inline-image behavior is intentionally not advertised beyond that.
• Large attachments on slow SMTP links may require increasing timeoutMs.

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