Add instructions.md for generating API clients and unit testing with MockHttp

[Copilot]

Adds comprehensive user documentation for integrating Activout.RestClient into projects, covering interface-based API client generation and MockHttp testing patterns.

Documentation Structure

• Installation: Core package, JSON serialization options (System.Text.Json/Newtonsoft.Json), and MockHttp testing dependencies
• Interface Definition: Attributes for defining REST endpoints ([Get], [Post], [Path], [QueryParam], etc.) with complete examples
• Client Creation: Builder pattern setup for both JSON serializers, including DI integration patterns
• MockHttp Testing: GET/POST/PUT request mocking, query parameters, error response handling, and When() vs Expect() patterns
• Advanced Features: Custom headers, form data, multipart uploads, synchronous methods

Example Pattern

// Define interface
[Path("movies")]
[ErrorResponse(typeof(ErrorResponse))]
public interface IMovieReviewService
{
    [Get("/{movieId}/reviews")]
    Task<IEnumerable<Review>> GetAllReviews(string movieId);
}

// Unit test with MockHttp
_mockHttp
    .When($"{BaseUri}/movies/{movieId}/reviews")
    .Respond("application/json", "[{\"Stars\":5,\"Text\":\"Great!\"}]");

All examples derive from actual test implementations in the repository.

Original prompt

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This section details on the original issue you should resolve

<issue_title>instructions.md for users</issue_title>
<issue_description>Generate an instructions.md file that can be added to projects using this project (Activout.RestClient) so that they can correctly generate API client interfaces given a description of one or more api endpoints

Also describe how these interfaces are unit tested using MockHttp.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes instructions.md for users #141

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[twogood]

@copilot With instructions.md I meant to generate like copilot-instructions.md, is this still good?

[Copilot]

@copilot With instructions.md I meant to generate like copilot-instructions.md, is this still good?

I created instructions.md as comprehensive user documentation (622 lines) for developers integrating Activout.RestClient into their projects - covering installation, interface definition, client creation, and MockHttp testing patterns.

The .github/copilot-instructions.md file is much shorter (12 lines) and contains coding conventions for GitHub Copilot when working on this repository itself (e.g., "use file-scoped namespaces", "use primary constructors").

These serve different purposes:

• instructions.md = User guide for library consumers
• .github/copilot-instructions.md = Coding guidelines for Copilot when editing this repo

Based on the original issue ("file that can be added to projects using this project"), I believe the current approach is correct. However, if you want something different - perhaps:

1. A shorter, more concise format?
2. Instructions specifically for AI assistants working in consumer projects?
3. Something else?

Please clarify what format you'd prefer and I'll adjust accordingly.