ci: build the .NET and Python onboarding examples

[mikemcdougall]

Summary

The README and docs/getting-started.md prominently advertise running all three example apps:

cd examples/dotnet && dotnet run
cd examples/python && pip install -r requirements.txt && python main.py
cd examples/javascript && npm install && npm start

But CI only built the JavaScript example (examples-javascript). The examples/dotnet and examples/python onboarding samples had zero CI coverage and could silently break against schema or API changes — the exact regression the JS-example job exists to prevent. (For instance, the dotnet example can drift to a different Grpc.Net.Client version than getting-started / the smoke test.)

Change

Add two cheap jobs mirroring examples-javascript:

• examples-dotnet — dotnet build examples/dotnet/GeospatialExample.csproj. The csproj compiles the protos directly via Grpc.Tools, so this is a restore+build with no buf/network codegen.
• examples-python — install requirements.txt, buf generate the Python stubs into examples/python (matching the documented generate → copy gen/python/* flow), then import-check main.py plus the generated modules it imports.

Both jobs are wired into the notify success gate.

Verification (local)

• dotnet build examples/dotnet/GeospatialExample.csproj --configuration Release → Build succeeded. 0 Warning(s) 0 Error(s).
• buf generate --template buf.gen.python.yaml --output examples/python → stubs generated; import-check prints python example imports resolve.
• ci.yml parses as valid YAML; job graph intact.

Finding addressed

• .github/workflows/ci.yml — .NET/Python examples advertised with run commands but never built/run in CI.