Replace Appsmith's mock server endpoint with MockWebServer

Editor assessment

This is a well-built fix message: the subject is specific about both the old and new mechanism, starts lowercase, uses the imperative, and stays close to the 72-character ceiling even with the PR reference attached. Naming MockWebServer gives future readers a searchable term when tracing test-infrastructure changes. There is no scope, which is a missed opportunity in a monorepo the size of Appsmith, and no body, but for a contained test-harness swap the header alone carries enough information. A patch bump is clearly implied.