chore: cortex chat enhancement

[louis-jan]

Describe Your Changes

• Improved the cortex chat command to be smarter:
  • cortex chat without model_id: looks for an active running model or selects a running model from the list.
  • cortex chat message and cortex chat -m message work the same (and do not look for a non-existent model with that input).
• Fixed an issue where Cortex is killed after a chat session ends.
• Fixed an issue where stopping a model also kills Cortex.

Fixes Issues

• Closes #
• Closes #

Self Checklist

• Added relevant comments, esp in complex areas
• Updated docs (for bug fixes / features)
• Created issues for follow-up changes or refactoring needed