ruby: run generate in parallel #7458

This issue tracks the implementation of parallelizing Ruby client library generation and cleanup during librarian generate --all.

Architecture Awareness

The implementation in internal/librarian/ruby/generate.go and internal/librarian/ruby/clean.go (and wired into internal/librarian/generate.go).

Steps

1. Parallelize library cleanup in internal/librarian/generate.go / internal/librarian/ruby/clean.go across available CPUs.
2. Ensure Ruby generation in internal/librarian/ruby/generate.go safely executes concurrently per library using errgroup with concurrency limits capped by runtime.NumCPU().
3. Separate generation and formatting phases if formatting requires all sibling gems to be present on disk.
4. Benchmark librarian generate -all in google-cloud-ruby to measure performance improvements.