Optimize ListConfigVariablesService to use Ci::Config directly

What does this MR do and why?

Optimizes Ci::ListConfigVariablesService by using Ci::Config directly instead of YamlProcessor to extract pipeline variables with prefill data.

Why: YamlProcessor performs unnecessary work for this use case - job normalization, validation, and dependency checking - when we only need to extract root-level variables.

Changes:

• Adds build_ci_config method that creates Ci::Config directly
• Adds feature flag optimize_list_config_variables to control the new behavior
• Refactors extract_variables to handle both Ci::Config and YamlProcessor::Result objects
• Removes unused verify_project_sha parameter from execute_yaml_processor
• Wraps existing specs in shared examples to test both code paths

#599883 (closed)

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