Make Ci::JobDefinition config size limit configurable

What does this MR do and why?

Since GitLab 18.11, the compiled per-job config stored in p_ci_job_definitions was subject to a hardcoded 1 MiB size limit enforced by Ci::JobDefinition. When a job's compiled config exceeded this limit, pipeline creation failed with Config is too large. Maximum size allowed is 1 MiB, with no way to raise the limit short of a code change.

Rather than introduce a new user-facing setting, this MR resolves the Ci::JobDefinition size_limit from the existing ci_max_total_yaml_size_bytes application setting. That setting already bounds the entire pipeline configuration, and a single job's config is always a subset of it, so a single job can never exceed the total YAML size. Reusing it avoids a second, overlapping limit that would be confusing to administrators.

JsonSchemaValidator now resolves a callable size_limit, allowing Ci::JobDefinition to read the value from application settings at validation time.

Self-managed administrators who need to raise the limit for workloads that legitimately require larger per-job configs can do so by raising the existing ci_max_total_yaml_size_bytes setting, without upgrading or patching GitLab.

References

• Resolves #602973

How to set up and validate locally

1. In the Rails console, lower the limit: ApplicationSetting.update(ci_max_total_yaml_size_bytes: 10)

2. Create a pipeline whose job config exceeds 10 bytes and confirm creation fails with a "too large" error. e.g.:

   test-job:
     variables:
       BIG_VALUE: "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"  # 50+ bytes
     script:
       - echo "$BIG_VALUE"

3. Raise the limit back to the default: ApplicationSetting.update(ci_max_total_yaml_size_bytes: 314572800)

4. Confirm the same pipeline now creates successfully.

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