fix: define UserResponse struct and update handler to use it

[filipe1309]

This pull request introduces a new UserResponse struct to standardize user data returned by the API handler. The handler now uses this struct for its response instead of a generic type, improving clarity and type safety.

API response improvements:

• Added a UserResponse struct to cmd/api/main.go to define the expected fields (ID, Email, Name) in user-related API responses.
• Updated the API handler in cmd/api/main.go to unmarshal and return responses using the new UserResponse struct instead of a generic type.

[Copilot]

Pull Request Overview

This PR introduces a structured UserResponse type to replace generic types in API responses, improving type safety and API contract clarity. The change standardizes the user data format returned by the handler.

• Added UserResponse struct with clearly defined fields for user data
• Updated handler to use the new struct instead of generic any type

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[github-actions]

📊 Test Coverage Report

Status: ✅ PASSED
Result: Coverage requirements met

🎯 Coverage Details:

"Total coverage threshold (80%) satisfied:\tPASS\nTotal test coverage: 98.5% (66/67)\n"

📋 Coverage Requirements:

• Minimum Threshold: 80%
• Coverage Profile: ./cover.out
• Coverage Mode: atomic

🔧 Test Configuration:

• Go Version: go version go1.25.1 linux/amd64
• Race Detection: enabled
• Timeout: 2 minutes
• Test Packages: ./...