Airflow 3.1+ uses OpenAPI 3.1's numeric exclusiveMinimum which
kin-openapi v0.131.0 cannot parse, causing silent failures and
"no endpoints found" errors. Replace with pb33f/libopenapi v0.34.0
which fully supports OpenAPI 3.0, 3.1, and 3.2.

Introduces self-contained types in pkg/openapi/types.go so the
library dependency is isolated to the parsing layer, making tests
simpler and future library swaps trivial.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>