Validate OpenAPI spec payloads

[bernardassistant557-arch]

Summary

• ensure OpenAPI spec parsing only returns mapping objects
• raise a clear error when the spec payload isn't a JSON/YAML object
• add a unit test that covers the invalid-body case

Problem

fetch_openapi_spec would happily return non-dict payloads (e.g., a plain string), which makes downstream usage fail later and produces confusing errors.

Reasoning

OpenAPI specs are defined as JSON/YAML objects. Enforcing that early keeps behavior predictable and surfaces bad specs immediately.

Changes

• validate parsed JSON/YAML is a dict before returning
• tighten the error message when parsing fails
• add a MockTransport test to guard against non-dict specs

Testing

• uv run pytest tests/test_datagouv_api.py (fails locally: uv not installed)

[bolinocroustibat]

Thanks for tightening the contract by only accepting a top-level object (dict) and adding a test for a 200 + non-spec body.

Could you provide an example when this happens?

Also, please solve the merge conflict with main branch.

[bolinocroustibat]

Let's keep a single-line raise to avoid unrelated formatting in the PR.