feat(3486): validate that requires only reference existing stages

[yk634]

Context

We found that a pipeline can currently accept a workflow where requires references a stage that does not exist in the YAML. When the UI tries to render a pipeline with that invalid workflow, it can crash.

This PR was motivated by that failure mode: invalid stage references should be rejected during config parsing instead of being allowed through to later parts of the system.

Example of invalid YAML that motivated this change:

jobs:
  job1:
    image: node:18
    requires: []
    steps:
      - echo: echo job1

  job2:
    image: node:18
    requires:
      - stage@missing-stage
    steps:
      - echo: echo job2

This configurations refer to stages that are not defined in stages, but they were not previously rejected by config validation.

Objective

This PR adds validation in config-parser so that requires can only reference stages that actually exist in the pipeline definition.

The change covers both of these cases:

• job-level requires
• stage-level requires

It also extracts normalizeRequires into a small shared helper so requires normalization is handled consistently across the stage validation and flattening paths.

References

screwdriver-cd/screwdriver#3486

License

I confirm that this contribution is made under the terms of the license found in the root directory of this repository's source tree and that I have the authority necessary to make this contribution on behalf of its copyright owner.