[moetemp]

What changed?

This PR adds the nexus.python_module file option, in a new proto/nexus/options.proto that the crate includes. A .proto file that sets it names the Python module of its messages and enums. A file that doesn't keeps the module that nexgen derives from its path.

import "nexus/options.proto";

option (nexus.python_module) = "streamprotos.temporal.api.stream.v1.message_pb2";

• prost_types::FileOptions drops extension fields, so nexgen reads the option from the raw descriptor bytes.
• nexgen refuses a value that isn't a dotted Python module name.
• The sample's three Stream protos import the options file and set the option. streams.bin, the Python and .NET bindings and the README's rebuild steps change with them.

In the stream sample's Python output, the models import the Stream protos from streamprotos.

Why?

By default, Python imports a temporal/... proto from temporalio, and the Stream protos of temporalio/api#895 aren't in a released temporalio. The module depends on where the protos are installed, so the protos name it, not each WIT binding. sdk-python doesn't need the option, since it generates those protos into temporalio itself.

How did you test it?

Link to a test plan if any -

• Unit Tests
• Staging
• End to End Tests

Backend unit tests cover the module choice and the read of the raw option, and a rejection fixture covers a bad module name. test_stream_service.py round-trips each read item kind through the streamprotos classes, and basedpyright checks the stream service.