Add value aliases to parallel:matrix job names

What does this MR do and why?

Adds an optional { value:, alias: } form for parallel:matrix values. The alias is used in the generated job name, while the CI/CD variable keeps the full value. This keeps job names readable and under the 255-character limit when matrix values are long, such as image references, URLs, or paths.

build:
  parallel:
    matrix:
      - OS: [ubuntu, alpine]
        IMAGE:
          - value: registry.example.com/builder-u20:0.0.3
            alias: u20
          - registry.example.com/builder-u22:0.0.1

This generates build: [ubuntu, u20], build: [ubuntu, registry.example.com/builder-u22:0.0.1], and so on. $IMAGE always contains the full value.

• needs:parallel:matrix finds the needed job by its full value or its alias. The value/alias hash form is rejected in needs.
• When aliases are used, matrices that generate duplicate job names are rejected. Matrices without aliases keep the current behavior.
• The CI JSON schema is not changed yet: the pipeline rejects the new form while the flag is disabled, so the editor should too. The schema update will follow with the rollout in #634481.
• The change is behind the ci_parallel_matrix_value_alias feature flag (beta, disabled by default). With the flag disabled, the hash form fails validation.

This builds on the patch that @nikhil_raman shared in the issue.

References

• Related to #285853
• Design agreed in #285853 (comment 1987955811)
• Related epic: &11791

Screenshots or screen recordings

Not applicable: no UI changes. Job names keep the existing job: [a, b] format.

How to set up and validate locally

1. In the Rails console, enable the feature flag:

   Feature.enable(:ci_parallel_matrix_value_alias)

2. Run a pipeline with the example from doc/ci/yaml/_index.md#parallelmatrix.
3. Check the job names, and check that $IMAGE contains the full value in each job.

MR acceptance checklist

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