Count a Duck.ai prompt toward the search experiment metric

[LukasPaczos]

Task/Issue URL: https://app.asana.com/1/137249556945/project/1208671518894266/task/1216793387267215?focus=true
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Description

Aligns expected retention metrics with other platform by counting Duck.ai prompts towards the search metric.

To avoid impacting in-flight experiments, any active experiment from the current local/remote config is excluded.

Steps to test this PR

Verify unit tests, and that they pass on CI.

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Note

Medium Risk
Changes when search retention pixels fire for experiments; a wrong or incomplete exclusion list could distort in-flight experiment metrics.

Overview
Duck.ai ATB refresh now also fires search retention metrics, alongside existing duck_ai_prompt_sent pixels, so Duck.ai prompt activity is counted toward search experiment retention like other platforms.

Search metrics are filtered on the Duck.ai path only: experiments listed in EXPERIMENTS_EXCLUDED_FROM_DUCK_AI_SEARCH_METRIC (currently addToDockAndWidgetExperimentJul25) are skipped so pre-existing in-flight experiments are not skewed. Search ATB refresh is unchanged—those experiments still get search metrics there.

Unit tests cover dual-metric sends on Duck.ai refresh, the exclusion on Duck.ai, and unchanged behavior on search refresh.

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[marcosholgado]

Added a couple of comments, not sure this needs to have this complexity.

[marcosholgado]

LGTM but let's wait to get the go ahead from DS just in case we also need to do something with atb.