POST /validate returns 500 instead of 400 when feedUrl is missing or field name is wrong

[jcpitre]

Created by Copilot.

Description

When a POST request is made to /validate with an incorrect field name (e.g. url instead of feedUrl), or with a missing feedUrl field entirely, the API returns a 500 Internal Server Error instead of a meaningful 400 Bad Request.

Steps to reproduce

curl -X POST https://gbfs.api.mobilitydatabase.org/validate \
  -H "Content-Type: application/json" \
  -d '{"url":"https://gbfs.velobixi.com/gbfs/gbfs.json"}'

Expected behavior

{ "status": 400, "error": "Bad Request", "message": "feedUrl is required" }

Actual behavior

{ "status": 500, "error": "Internal Server Error", "path": "/validate" }

Root cause

In ValidateApiDelegateHandler.validatePost(), getFeedUrl() returns null when the field is absent or misnamed. This null is passed directly to loader.load(), which throws an uncaught exception that Spring converts to a 500.

Suggested fix

Add a null/blank check at the top of validatePost():

if (validatePostRequest.getFeedUrl() == null || validatePostRequest.getFeedUrl().isBlank()) {
    return ResponseEntity.badRequest().build();
}

Or preferably, annotate feedUrl with @NotBlank in ValidatePostRequest and add @Valid to the controller parameter — Spring will then return a 400 automatically with a descriptive message.