chore: onboard google-cloud-capacityplanner to librarian

[quirogas]

Description
This library cannot currently be onboarded because automated generation deletes the legacy CapacityPlanningServiceClient, causing existing unit tests to fail.

Historical Context & Investigation

During a local OwlBot regeneration attempt (google-cloud-node#8642), CapacityPlanningServiceClient vanished from the generated client code, breaking the unit test suite.

As analyzed by @jskeet in #6495, CapacityPlanningService was officially deleted from upstream protobuf definitions in October 2025 (googleapis commit 2954ae6), leaving only UsageService. Consequently, the deletion of CapacityPlanningServiceClient is expected, and legacy test files require updating before onboarding can proceed.

Additionally, as noted by @jskeet in google-cloud-node#8643 (comment), capacityplanner might share post-generation package.json metadata stabilization requirements with google-monitoring-dashboard.

Librarian Version: v0.20.0
librarian.yaml Permalink: Link to librarian.yaml

Reproduction Steps

Provide the exact commands executed that led to the failure:

docker run --rm \
  -v $(pwd):/repo \
  -v ~/.cache:/.cache \
  -w /repo \
  localhost/librarian-nodejs:v0.20.0 \
  generate -v google-cloud-capacityplanner

Additional Information & Resources

• Relevant Pull Requests / Issues: google-cloud-node#8642, google-cloud-node#8643 (comment), nodejs: google-cloud-capacityplanner fails tests with OwlBot update #6495