Fix incident severity null deserialization

[kwent]

Summary

• allow Incident.severity to deserialize and serialize explicit JSON null values
• add an end-to-end regression test through IncidentResponse.from_dict()
• run the regression suite across the existing Python version matrix
• document the fix in the changelog

The corrected OpenAPI schema is deployed, but a full regeneration with openapi-python-client 0.26.2 currently drops the severity endpoints after detecting duplicate response shapes. This focused generated-model patch fixes the production failure without removing those APIs or including unrelated generated changes.

Verification

• make test
• uvx --from ruff==0.16.3 ruff check .
• uvx --from ruff==0.16.3 ruff format --check .

Fixes #18