fix(email): add email SMTP STARTTLS fallback

[hxp-plus]

What does this PR do?

Fixes SMTP connection handling in the email gateway so both implicit TLS on port 465 and STARTTLS on port 587 work correctly.

Previously, the adapter expects a plain SMTP connection upgraded with STARTTLS on port 587 and can not handle SMTP servers with port 465 with SMTP SSL. This PR unifies SMTP connection paths behind a single flow:

1. Probe whether the target SMTP server supports STARTTLS
2. If STARTTLS is supported, connect with smtplib.SMTP(...) and upgrade via starttls(...)
3. Otherwise, connect with smtplib.SMTP_SSL(...)

This keeps 587-compatible servers working while preserving support for 465-only SMTP setups.

Related Issue

Fixes #11842

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• Unified SMTP connection setup in gateway/platforms/email.py
• Added STARTTLS capability probing before selecting the SMTP transport
• Updated all 3 SMTP call sites to use the shared connection logic:
  • SMTP connection test in connect()
  • Message sending in _send_email()
  • Attachment sending in _send_email_with_attachment()
• Added regression coverage in tests/gateway/test_email.py for:
  • STARTTLS-capable SMTP servers
  • SMTP_SSL fallback when STARTTLS is unavailable
  • Send, send-with-attachment, and connect flows

How to Test

1. Configure the email gateway with an SMTP server that uses STARTTLS on port 587 and verify connect() and message sending still work
2. Configure the email gateway with an SMTP server that uses implicit TLS on port 465 and verify connect() and message sending work via SMTP_SSL
3. Run:
   • scripts/run_tests.sh tests/gateway/test_email.py -q
   • scripts/run_tests.sh tests/gateway/ -q

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/ -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: Linux

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

Screenshots / Logs

N/A

[alt-glitch]

Competing fix with #13564 which also adds SMTP port 465 support. Both address #11842.

[hxp-plus]

This issue is solved in lateset code. Closing the issue.