Add validation for missing project/user in Ci::CreatePipelineService

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Summary

Following up on the discussion in !200986 (merged), we should add validation for missing projects and users in Ci::CreatePipelineService as well, in addition to the existing guards in CreatePipelineWorker.

Background

In !200986 (merged), we added guards in CreatePipelineWorker to handle missing projects and users gracefully:

project = Project.find_by_id(project_id)
return unless project

user = User.find_by_id(user_id)
return unless user

Proposal

@allison.browne suggested that we should also add validation in the Ci::CreatePipelineService itself. The benefits would be:

1. Better error handling: We could use error() without a failure reason to populate ::Ci::PipelineCreation::Requests with an error, allowing users to see why the latest pipeline for a project or MR wasn't created.

2. Broader protection: This would handle the condition from other callers of Ci::CreatePipelineService, not just the worker.

3. Consistent behavior: Using error(msg) in the pipeline creation service without a failure_reason will not persist the pipeline, having the same effect as not creating a pipeline.

Implementation Notes

• Add validation in Ci::CreatePipelineService to check that both project and user exist
• Use the service's error() method to provide meaningful feedback when they don't exist
• Ensure the error doesn't persist a failed pipeline (no failure_reason)
• Keep the existing validation in CreatePipelineWorker as well

Related

• Original MR: !200986 (merged)
• Discussion: !200986 (comment 2683423302)