Add granular PAT support for PipelineExecutionProjectScheduleType

What does this MR do and why?

Enables granular personal access token (PAT) authorization for the PipelineExecutionProjectScheduleType GraphQL type.

The authorize_granular_token directive was already present on the type, but authorization was not working because the policy_management_project boundary method only existed on the GraphQL type wrapper. The GranularScopeAuthorization class calls boundary methods on the unwrapped model object, so the method needed to be moved to the model.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist.

How to set up and validate locally

1. Create a pipeline execution schedule policy in a project
2. Generate a granular PAT with read_pipeline_execution_project_schedule permission scoped to the policy management project
3. Query project.pipelineExecutionSchedulePolicies.upcomingSchedules using the granular PAT
4. Verify access is granted with the correct permission and denied without it

Related to

Closes #602343 (closed)