fix(email): use SMTP_SSL for port 465 and fall back to IPv4 on timeout

[liuhao1024]

What does this PR do?

Fixes the Email gateway adapter to use SMTP_SSL (implicit TLS) for port 465 connections and adds IPv4 fallback when IPv6 is unreachable. Previously, the adapter always used SMTP() + starttls(), which is correct for port 587 but causes hangs/failures on port 465 providers.

Related Issue

Fixes #46018

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• gateway/platforms/email.py: Extracted _connect_smtp() helper that selects SMTP_SSL for port 465 and SMTP + STARTTLS for other ports. Added IPv4 fallback when the default connection times out (handles hosts with AAAA records on networks without IPv6 routing). Replaced 4 duplicate SMTP connection sites with calls to the helper.
• tests/gateway/test_email.py: Added TestConnectSmtp class with 4 tests: port 587 uses SMTP+STARTTLS, port 465 uses SMTP_SSL, IPv6 timeout falls back to IPv4 for port 587, and IPv6 timeout falls back to IPv4 for port 465.

How to Test

1. Configure Email gateway with a port 465 provider (e.g., EMAIL_SMTP_PORT=465)
2. Start the gateway — SMTP connection should succeed using SMTP_SSL
3. Configure with a host that has AAAA records but no IPv6 routing — connection should fall back to IPv4 within the timeout
4. Run pytest tests/gateway/test_email.py -x -q — all 66 tests pass

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/gateway/test_email.py -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: macOS

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

Code Intelligence

• Analyzed: gateway/platforms/email.py (_connect_smtp, _send_email, _send_email_with_attachments, _send_email_with_attachment, connect)
• Blast radius: LOW — changes are confined to SMTP connection establishment; send logic and message formatting are unchanged
• Related patterns: IMAP already uses imaplib.IMAP4_SSL (implicit TLS) — this fix brings SMTP to parity

[teknium1]

Merged via PR #46084: #46084

Your SMTP_SSL port-465 fix was cherry-picked onto current main with authorship preserved. We added a follow-up to make the IPv4 fallback use supported smtplib extension points, avoid global resolver monkeypatching, and preserve TLS verification failure semantics.

Thanks for the fix.