Add OpenAPI "drift detecting" test

[Kostis-S-Z]

Add a daily CI action that:

1. pulls the latest OpenAPI spec from the platform
2. runs the test to detect drifts of the API contract between the SDK and the platform
3. if drift is detected it opens a PR with the result in the body

The actual tests/test_openapi_contract.py reads the openapi spec and checks that:

• every endpoint the SDK calls exists with the same method
• the Task and Visibility enums match what the update request accepts
• the draft, update and submit payloads match the request schemas (update and submit in both directions, so a field the platform accepts but the SDK cannot set fails too)
• every field on DatasetDetails exists in the dataset response, and every field on DatasetSubmission is either returned on a submission or accepted on PATCH
• the upload and download payloads match, and the response keys the SDK relies on are marked required.

When the spec and the SDK disagree, the test will be red until the SDK model is fixed.
Can also use MDC_OPENAPI_SPEC to point it at a different spec (file or URL)