fix: filter fastapi-only params

[francisdbillones]

What

I explicitly defined the kwargs we're passing with fixed values into Air's internal FastAPI instance, and made the app factory raise a ValueError that points them to the fastapi_app route if they try to pass their own values instead.

Fixes issue #1073

Pattern

Followed the existing app factory pattern.

Reviewer Focus

I don't think any special attention is required from the reviewers.

Checklist

• Diff contains only changes for this task — no unrelated refactoring or cleanup
• Addresses exactly one issue or feature
• New or changed behavior has test coverage
• This is the simplest viable approach
• AI provenance section removed or accurate

[pydanny]

Hey @francisdbillones, thanks so much for this PR. It's a nice catch that will help on these sorts of cases. I have one question about a single line of code, I look forward to a better understanding. 😄

[pydanny]

Why are we testing fastapi_kwargs_with_fixed_values here?