fix: correct template list command hint

[Jackchen-1145]

Summary

• Fix the template-not-found error message to suggest fenn list instead of the unsupported fenn pull --list.
• Add regression assertions so the CLI output keeps pointing users to the valid command.

Validation

• Ran Python syntax check:
  python -m py_compile fenn/cli/pull.py tests/unit/cli/test_pull_command.py

Full pytest was not run locally because this environment does not have pytest installed.

[ApusBerliozi]

Hey @Jackchen-1145!

Thx 4 your work! Kindly check why pre-commit job is failing