Fix proto generation for new subdirectories

[rkannan82]

What

Replace mv -f with cp -rf when flattening generated proto output, so new subdirectories (e.g. nexusservices) don't cause the build to fail.

Why

mv cannot merge directories — when a new proto subdirectory is added to the api repo, mv fails with "cannot overwrite: Directory not empty". This is what's breaking the Update Proto workflow.

Example error:

Submodule path 'proto/api': checked out '67150b14e0509210bf250960bd3278a4509e091c'
Deleting generated go files...
Installing protoc plugin
Compile for go-gRPC...
mv: cannot overwrite './nexusservices': Directory not empty
make: *** [Makefile:56: go-grpc] Error 1
Error: Process completed with exit code 2.

How did you test it?

Triggered a test run and it succeeded.
https://github.com/temporalio/api-go/actions/runs/24223347639

  gh workflow run update-proto.yml --repo temporalio/api-go --ref kannan/fix-makefile-mv \
    -f branch=kannan/fix-makefile-mv \
    -f commit_author="rkannan82" \
    -f commit_author_email="kannan@temporal.io" \
    -f commit_message="Test: verify proto generation with mv fix"

🤖 Generated with Claude Code

[CLAassistant]

All committers have signed the CLA.