tplink: model fix and added TP-Link DeltaStream GPON OLT unit tests and model notes

[Vantomas]

Pre-Request Checklist

• Passes rubocop code analysis (try rubocop --auto-correct)
• Tests added or adapted (try rake test)
• Changes are reflected in the documentation
• User-visible changes appended to CHANGELOG.md

Description

Add model unit tests and model notes for the tplink model, validated against a TP-Link DeltaStream DS-P7001-08 GPON OLT (software 1.0.0), plus a small pre_logout fix.

Changes

• tplink pre_logout: Send \r\n in pre_logout. This is what lets the captured simulation match the model in the unit test.
• Model unit tests (spec/model/data/tplink#...): simulation, expected output, prompt and secret fixtures for the DeltaStream DS-P7001-08.
• Model notes (docs/Model-Notes/TPLink.md):
  • SSH auth caveat — some TP-Link devices abort the SSH connection if the client offers a public key before password, so an Oxidized host that has SSH keys gets the connection closed before the password is tried. Fix with auth_methods: ["none", "password"].

Note on the simulation file: it starts at the privileged # prompt with the enable step commented out (explained in a header comment in the file).