feat: fix name function add on merchantEndpoint

[Benjamin-Freoua-Alma]

Reason for change

Linear task
Function add on MerchantEndpoint is named Add.
We can have error is the case is strict in the merchant env

Code changes

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Checklist for authors and reviewers

• The title of the PR uses business wording, not technical jargon, for the changelog readers to understand it
• The PR implements the changes asked in the referenced task / issue
• The automated tests are compliant with the testing strategy
• The tests are relevant, and cover the corner/error cases, not only the happy path
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• Documentation is updated (API, developer documentation, ADR, Notion...)

Non applicable