bug: openshell.spec packages Python protobuf stubs without generating them

[politerealism]

Part of: #3511

User Story

As a Fedora packager building the python3-openshell subpackage, I want the Python protobuf stubs to be generated as part of the build, so the package doesn't silently ship stale or missing files.

Problem Statement

python/openshell/_proto/*.py (the generated gRPC/protobuf stubs consumed by the Python SDK) is .gitignored and produced by tasks/scripts/generate_python_proto.py, not committed to the repo. openshell.spec's %install copies python/%{name}/_proto/*.py into the buildroot, but nothing in %prep/%build generates these files first.

Impact / Why This Matters

Without this fix, the python3-openshell subpackage either fails to build from a clean source tree, or worse, silently packages stale stubs left over from an unrelated local build — a correctness risk, not just a build failure.

Acceptance Criteria

• %prep or %build runs tasks/scripts/generate_python_proto.py (or equivalent) against the vendored proto/*.proto files, offline
• New BuildRequires added for whatever protoc/grpc-tools package this needs (e.g. python3-grpcio-tools or a protobuf-compiler equivalent)
• Build succeeds from a clean checkout with no pre-existing generated files
• %check verifies the generated stub files exist and are importable (mirroring the existing Python SDK smoke test already in %check)

Reproduction Steps

1. Check out a clean tree (or extract a git archive-style source tarball) with no prior local build artifacts.
2. Run rpmbuild -bb openshell.spec.
3. %install's install -pm 0644 python/%{name}/_proto/*.py ... fails — the files don't exist.

Environment

• Any environment building from a clean checkout/source tarball rather than a local dev tree that has already run proto generation

[politerealism]

Consolidating this into the parent tracking issue #3509 as a listed work item rather than keeping it as a separate issue. Closing this one — see #3509 for the current text.