❤️ TEST-#37: Add coverage for resolve_smtp_host (known + DNS + fallback)

[FernandoCelmer]

Summary

Expanded tests/test_providers_smtp.py from 6 cases to 26 covering every branch in SMTP host resolution:

• All KNOWN_SMTP_PROVIDERS aliases (gmail/googlemail, office365 family, icloud family, yahoo/ymail, zoho, hostinger, fastmail, aol)
• _split_email rejection paths: missing @, missing local part, missing domain
• _lookup_smtp_srv: no-dns short-circuit, DNS exception, SRV→465 SSL, SRV→587 STARTTLS
• _lookup_smtp_mx: no-dns short-circuit, DNS exception, hint match (aspmx.l.google.com → smtp.gmail.com), no-hint match returns None
• Resolution order: known provider short-circuits DNS, SRV wins over MX for unknown domain
• Convention fallback (smtp.<domain> on 465/SSL) when DNS unavailable
• Module surface: SMTP_MX_HINTS populated, ProviderResolutionError subclass

Closes #37

Test plan

• pytest tests/test_providers_smtp.py — 26 passed

[FernandoCelmer]

🔍 Code Review

Code issues found: 0

Test-only PR. Coverage matrix for resolve_smtp_host is now exhaustive:

• All KNOWN_SMTP_PROVIDERS aliases (gmail/googlemail/o365 family/icloud family/yahoo/ymail/zoho/hostinger/fastmail/aol)
• _split_email rejection branches
• _lookup_smtp_srv: no-DNS short-circuit, exception, SRV→465 (SSL), SRV→587 (STARTTLS)
• _lookup_smtp_mx: hint match + no-hint-match + DNS exception
• Resolution order asserts (known wins over DNS, SRV wins over MX)

_Target / _srv / _mx helpers mirror the dnspython shapes cleanly. No production code touched; risk is zero.

Closes #37.