fix: allow null position in Mapping for disabled mappings

[Copilot]

The OneLogin API returns null for position on disabled mappings, but position was typed as required StrictInt, causing a pydantic ValidationError on deserialization. Passing the existing position value as a workaround then triggered a 422 from the API ("Position cannot be set while the mapping is not enabled").

Changes

• onelogin/models/mapping.py: Relax position from StrictInt = Field(...) to Optional[StrictInt] = Field(None, ...). The existing exclude_none=True in to_dict() already omits None positions from serialized request bodies, so disabled-mapping updates no longer trigger the 422.

• test/test_mapping.py: Add tests covering position=None round-trip: deserialization from an API response with "position": null, deserialization when position is absent, and confirmation that to_dict() excludes a None position.

Example

# Previously raised pydantic ValidationError
mapping = Mapping.from_dict({
    "id": 42,
    "name": "My Mapping",
    "enabled": False,   # disabled → API returns null for position
    "match": "all",
    "position": None,   # ← was rejected by StrictInt
    "conditions": [...],
    "actions": [...],
})

# to_dict() correctly omits position=None from the update body
assert "position" not in mapping.to_dict()