FastAPI form routes fail at request time, not import time, without python-multipart

A route that declares `Form(...)` or calls `request.form()` imports cleanly and starts cleanly when `python-multipart` is missing. The failure only appears on the first request that reaches it.

Repro:

1. Create a FastAPI app with a route that takes form data: `async def hook(x: str = Form(...))`. 2. Run `pip uninstall python-multipart`. 3. Start the app. No error at import, no error at startup. 4. POST form data to the route, for example `curl -X POST -d "x=1" http://localhost:8000/hook`. 5. The response is a 500. The server log shows: `Form data requires "python-multipart" to be installed.`

The check is deferred until FastAPI has to parse a form body. Declaring the route does not trigger it, so nothing at import time or in a health check tells you the dependency is missing.

### Why this bites with Twilio

Twilio webhooks are sent as `application/x-www-form-urlencoded`, not JSON. A Twilio-facing endpoint is therefore a form endpoint, and it needs `python-multipart` to parse the body.

If your unit tests post JSON bodies to your own routes, they never exercise form parsing. The suite stays green, the deploy looks healthy, and the first real Twilio call returns a 500. That is the worst place to find out, because the caller is on the line and Twilio's retry and error handling is now your problem.

### What to do about it

- List `python-multipart` explicitly in your requirements. Do not rely on it arriving as a transitive dependency of something else. - Add at least one test per webhook route that posts real form-encoded data, for example with the test client's `data={...}` argument rather than `json={...}`. That test fails in an environment without the package, which is the point. - Run the tests in a clean virtualenv built from your requirements file, not in the environment you have been installing into for weeks. A stale environment can mask a missing pin. - If you want a startup-time guarantee, add an explicit import of `multipart` in your app module, so a missing package fails the process at boot rather than on a request.

The general lesson is narrow: a framework that defers an optional-dependency check to request time makes import success meaningless as a signal for that code path. Tests have to send the same content type production will send.