Enhance CPS onRequest handler

[gsoldevila]

Summary

Improves the getOnRequestHandler in ElasticsearchService with two behavioral changes:

• When CPS is disabled, the handler now actively strips any project_routing present in body, params.querystring, or options.querystring to prevent ES request failures. This avoids plugins having to check if CPS is enabled for every API call that might inject project_routing.
• When CPS is enabled, the handler now checks all three locations (body, params.querystring, options.querystring) for caller-provided project_routing before injecting the default value, avoiding unintended overrides.

Tests updated to use setCpsFeatureFlag via the setup contract instead of accessing private properties, and expanded to cover the new stripping and querystring scenarios.

Made with Cursor

[elasticmachine]

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[gsoldevila]

Handled in #253697