Bad Studio inverse field no longer takes the server down

Commit message

[FIX] core: studio crashes server when having invalid key in relational_field

Current behavior:
When a Studio custom One2many field has an invalid relation field
(inverse) name, accessing registry.field_inverses raises KeyError. On
19.0 this also crashes error-page rendering, leaving the server unusable
after closing Studio. On 18.0, it doesn't crash the server.

Expected behavior:
An invalid inverse name must not crash the registry or error handler;
the server should stay responsive so the user can fix the field.

Steps to reproduce:
1. Install sales, project with demo data
2. Create a sale order, use Studio to add a new "Lines" field
(e.g. test line)
3. In the sale order, add a product that has Create on Order field
(e.g. plumbing services from the demo data) > Confirm
4. Click on the smart button to go to project/task
5. Switch to Studio again, add a related field in the task, point that
to Sales Order > test line
6. Activate debug mode, click on More for test line (task)
7. In properties, turn off Read only. Important here: add an invalid key
for Relation Field (e.g. 'hehe')
8. Error popup shows key error. Try to close and exit out of Studio mode
> Server crashes with 500 Internal Server Error

Cause of the issue:
- When a custom One2many field has an invalid inverse_name (set via
Studio developer mode), One2many.setup_inverses accessed
registry[comodel]._fields[inverse_name] without checking existence
- Building the cached registry.field_inverse property then raises an
uncaught KeyError and bricks the server with 500 Internal Server Error

Fix:
- Add a constraint on ir.model.fields so a custom One2many cannot be
saved with a relation field that does not exist on the comodel
- Skip a missing inverse for manual fields in setup_inverses, so
databases that already store an invalid value will no longer crash

opw-6479178