Fix raw JSON validation errors on endpoint creation

[morcen]

What was broken

The "Create Endpoint" modal's save handler (resources/js/Pages/Endpoints/Index.vue) submits via a raw axios.post() call instead of Inertia's form.post(), because it needs to read the one-time plain_secret back from the JSON response on success. This bypasses Inertia's normal client-side error handling, which flattens Laravel's validation error response.

Laravel's default JSON validation-error response shapes each field as an array of messages, e.g. { "name": ["The name field is required."] }. The catch handler assigned this raw array straight into form.errors via form.setError(error.response.data.errors). InputError.vue expects its message prop to be a String, so Vue rendered the array via JSON.stringify, showing users a raw blob like ["The name field is required."] instead of a readable message — on every validation failure when creating an endpoint (duplicate name, invalid/unsafe URL, etc.).

What changed

• Flatten each field's error array down to its first message before calling form.setError(), matching the error shape Inertia's own form submissions already produce elsewhere in the app.
• Added tests/Feature/EndpointCreateValidationErrorFormatTest.php, a regression test pinning down the JSON validation-error response shape (array of messages per field) that the frontend fix depends on for the AJAX endpoints.store route.

Testing

• vendor/bin/pint --dirty — passes
• composer test (full PHPUnit suite) — 277 passed, 1 skipped, 0 failed

Fixes #140