Validate serverless pipeline cluster settings

[50sotero]

Changes

Reject non-empty pipeline cluster settings when serverless compute is enabled. The shared validator reports the configuration location and how to fix it during bundle validation, planning, and deployment.

Why

Fixes #1688. This invalid combination previously passed bundle validation and failed only when the pipeline was deployed. Classic pipelines and serverless pipelines with omitted or empty cluster settings remain valid.

Tests

• Confirmed the new acceptance test fails before the fix because the invalid configuration passes validation.
• Acceptance test passes for all three configured Terraform/direct/DMS variants, covering validation, planning, deployment rejection, and valid classic/serverless configurations.
• go test ./bundle/config/validate passes.
• Full Linux validation with this PR and Fix bundle file resync after remote directory deletion #6661 applied: ./task fmt, ./task checks, ./task lint, and ./task test pass. Results: 10,030 root-module unit cases (47 skipped), 73 tools-module cases, and 5,138 acceptance cases (11 skipped), with no failures. Go 1.26.8; acceptance uses the local test server.
• Changelog validation also passes with this PR's link. Live workspace integration tests were not run; upstream CI requires maintainer approval.

This PR was written by OpenAI Codex.

[github-actions]

An authorized user can trigger integration tests manually by following the instructions below:

Trigger:
go/deco-tests-run/cli

Inputs:

• PR number: 6662
• Commit SHA: dd62c51366a996fb643e69e2dd341068f72a2a91

Checks will be approved automatically on success.

[github-actions]

Approval status: pending

/acceptance/bundle/ - needs approval

5 files changed
Suggested: @denik
Also eligible: @shreyas-goenka, @pietern, @andrewnester, @janniklasrose, @lennartkats-db, @anton-107

/bundle/ - needs approval

Files: bundle/config/validate/fast_validate.go, bundle/config/validate/pipeline_cluster_spec.go
Suggested: @denik
Also eligible: @shreyas-goenka, @pietern, @andrewnester, @janniklasrose, @lennartkats-db, @anton-107

General files (require maintainer)

Files: .nextchanges/bundles/validate-serverless-pipeline-clusters.md
Based on git history:

• @denik -- recent work in .nextchanges/bundles/, bundle/config/validate/

_{Any maintainer (@andrewnester, @anton-107, @denik, @pietern, @shreyas-goenka, @simonfaltum, @renaudhartert-db, @janniklasrose, @lennartkats-db, @rugpanov, @rclarey) can approve all areas.
See OWNERS for ownership rules.}

[50sotero]

@denik, could you take a look at this fix for #1688 and help authorize the pending CI/integration tests? It rejects incompatible serverless pipeline cluster settings during validation, planning, and deployment, with coverage for valid classic and serverless configurations. Local formatting, checks, lint, and the full unit/acceptance suites pass; live workspace integration tests still need an authorized run.