The Python SDK had no pull_request-triggered checks, so regen PRs
(speakeasy-sdk-regen-*) carried an empty status-check rollup and the
auto-merge flow would merge them with zero validation. The TypeScript
SDK validates every regen PR via pr-validation.yaml; Python had no
equivalent.
Add a single-file PR Validation workflow that builds the package and
runs the SDK's existing quality gates (mypy, pyright, pylint) via uv.
No tests exist in this repo yet, so no API-key secret is needed, which
also lets the workflow run on fork PRs. paths-ignore on
.speakeasy/in.openapi.yaml mirrors TS so pure spec-bump PRs don't
double-trigger.
This registers a `validate` check that the auto-merge wait_for_checks
step will gate on once both land.
