fix(python): rewrite generated imports with protoletariat

[scarmuega]

Problem

The protobuf Python plugin emits absolute imports rooted at the proto package:

from utxorpc.v1alpha.cardano import cardano_pb2

These do not resolve once the code is packaged under utxorpc_spec (modules install as utxorpc_spec.utxorpc.*), so import utxorpc_spec... fails with ModuleNotFoundError: No module named 'utxorpc.v1alpha' — utxorpc-spec 0.19.x is un-importable.

generate.yml already had a Fix python imports step, but it used BSD sed -i '' syntax which is invalid on the GNU/Linux runner — it silently never applied.

Fix

Replace the sed hack with protoletariat, which rewrites proto-generated imports to relative imports by inspecting the FileDescriptorSet — location-independent and correct for both v1alpha and v1beta. --exclude-google-imports leaves the well-known-type imports alone.

Also adds an import smoke-test (v1alpha + v1beta) so an un-importable package can't ship silently again.

Follow-up

Needs a new spec release (v0.19.2) to republish a working utxorpc-spec; then python-sdk#9 bumps to it.

[scarmuega]

Verified by dispatching the standalone generate.yml on this branch (run 26179054318) — both Fix python imports (protoletariat) and Smoke-test python imports pass; import utxorpc_spec.utxorpc.v1alpha.sync.sync_pb2 and the v1beta equivalent both resolve.

Note: the pr-check.yml check on this PR shows startup_failure, but that is pre-existing and unrelated — every pr-check.yml run since 2026-05-19 fails the same way (on unrelated branches too). Cause: pr-check.yml declares permissions: contents: read, but it calls publish-all.yml whose publish-node job requires id-token: write; a reusable workflow cannot exceed the caller's permissions, so the run cannot start.