fix(pgp): require real PGP/MIME structure for encryption gate

[KamyarAT]

Summary

The PGP-only gate accepted any message containing the raw substring application/pgp-encrypted, including when that token only appeared in Subject, body text, MIME parameters, or other non-structural places. Attackers could therefore submit cleartext (or weakly structured) mail that still passed the encryption check.

This PR requires a real PGP/MIME structure: root Content-Type: multipart/encrypted and a part with Content-Type: application/pgp-encrypted (RFC 3156 control part). Secure-Join multipart/mixed handshakes and mailer-daemon + multipart/report bounces remain allowed.

Regression coverage from the earlier marker suite is restored and expected to pass with the production fix.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing behavior to change)
• Documentation only
• Build / CI / tooling
• Refactor (no functional change)

Area

• SMTP
• IMAP
• Federation / delivery queue
• Authentication / JIT
• Admin API / admin web UI (external/madmail-admin-web)
• TURN / Iroh / proxy services
• Storage / quota / DB migrations
• Configuration / CLI
• Docs (docs/project/, docs/TDD/, user guide)
• Other: chatmail-pgp encryption gate

Testing

• cargo fmt --all -- --check
• cargo clippy -p chatmail-pgp -p chatmail-smtp --all-targets -- -D warnings
• cargo test -p chatmail-pgp --lib (30/30)
• cargo test -p chatmail-smtp --lib (37/37, includes marker → 523 and real PGP/MIME → 250)
• Manual / E2E verification (describe below)

Manual verification (if any):

cargo test -p chatmail-pgp --lib
cargo test -p chatmail-smtp --lib smtp_rejects_pgp_marker
cargo test -p chatmail-smtp --lib smtp_still_accepts_real_pgp_mime

Documentation

• No doc updates needed
• Updated operator/user docs (docs/project/user-guide/, install guides)
• Updated developer docs (docs/project/)
• Updated or added TDD section (docs/TDD/)

Security & privacy

• Not security-sensitive
• Security-sensitive — added/updated tests for the security property
• Reviewed error messages for information leakage

Failures still surface as 523 Encryption Needed / EncryptionNeeded without leaking internal structure details.

Commits

• fix: bug fix

Checklist

• PR is focused and reviewable (small vertical slices preferred)
• No secrets, data/, target/, or node_modules/ committed
• Submodule pointers updated if external/madmail-admin-web changed
• Behavior parity with Madmail v1 considered (or intentional difference documented)

Aligns with TDD/security policy: properly formed multipart/encrypted PGP/MIME, Secure-Join, and bounce exceptions only.

[omidz4t]

I think this must be a feature that we can activate it on the dashboard panel as an service option to enforce it hardy + know the cost of it