Update completion model fields immediately on save

[dlqqq]

Description

• Follow-up to Update model fields immediately on save #1125. Fixes an issue identified by @krassowski.
• Fixes completion model fields by reading from the correct config key when returning model arguments for a completion model.
• Adds unit test coverage for verification.

Testing instructions

1. Checkout this branch, and make the following changes near line 459 in config_manager.py to mimic the existing behavior on main:

           # get config fields (e.g. base API URL, etc.)
   -       if completions:
   -           fields = config.completions_fields.get(model_uid, {})
   -       else:
   -           fields = config.fields.get(model_uid, {})
   +       fields = config.fields.get(model_uid, {})

2. Run pytest, verify that the new unit test fails.

3. Revert your local changes, re-run pytest, and verify that the new unit test passes.

[srdas]

Tested the changes to config_manager.py -- tests pass after the change.

Code changes LGTM.

[krassowski]

LGTM, thank you @dlqqq!

[dlqqq]

@meeseeksdev please backport to v3-dev