Validate optional integration credentials before creating transport

When an optional Python integration is created lazily inside a nonblocking business operation, validate required credential values before constructing its HTTP transport. Raise a typed integration configuration error that the existing caller failure boundary can return and record as failure. Direct environment indexing instead produces an unexpected KeyError, while empty values can survive until authentication. Resolve explicit arguments using None as the fallback signal so an explicitly blank override cannot silently use environment credentials. Check blankness without rewriting nonblank secret bytes. This establishes safe missing-configuration behavior, not credential validity or remote acceptance.

Applies when: lazy integrations should fail observably without blocking the surrounding operation. Keywords: Python configuration credentials constructor httpx validation KeyError transport