fix: show doctor remediation for unfixed findings

[immanuwell]

Small paper cut fix.

cog doctor --fix could print (no auto-fix available) even when the finding already had a remediation message. So the command knew what to say, it just wasnt showing it.

Repro:

1. Make a cog.yaml with:

   build:
     python_version: "3.12"
   predict: "predict.py:Predictor"

2. Do not create predict.py
3. Run cog doctor --fix
4. Before this patch, Predict reference ended with (no auto-fix available)
5. Now it prints the actual remediation, like Create predict.py or update the predict field in cog.yaml

Also adds a unit test and a small integration test, nice and tidy.