Fix CancellationToken body parameter bug for POST/PUT/PATCH methods

[Copilot]

This PR fixes a bug where CancellationToken parameters were incorrectly being used as the request body for POST, PUT, and PATCH HTTP methods.

Problem

When a REST client method had a CancellationToken as the last parameter, the RequestHandler would attempt to serialize it as the request body instead of using the actual body parameter. This caused serialization errors like:

System.NotSupportedException: Serialization and deserialization of 'System.IntPtr' instances is not supported. Path: $.WaitHandle.Handle.

Root Cause

The RequestHandler constructor always set _bodyArgumentIndex to _parameters.Length - 1 (the last parameter), without considering that the last parameter might be a CancellationToken that should be handled separately.

Solution

Added logic in the RequestHandler constructor to detect when:

1. The last parameter is a CancellationToken
2. The HTTP method supports request bodies (POST, PUT, PATCH)

In these cases, the body argument index is adjusted to use the second-to-last parameter (_parameters.Length - 2) instead.

Example

Before this fix, the following would fail:

public interface IApiClient
{
    [Post("/api/data")]
    Task CreateData(MyRequest data, CancellationToken cancellationToken);
}

Now it correctly serializes data as the request body and uses cancellationToken for cancellation.

Testing

Added comprehensive tests covering:

• POST, PUT, and PATCH methods with CancellationToken as last parameter
• Verification that the correct request body is sent (not the CancellationToken)
• Regression test ensuring methods without CancellationToken still work

All existing tests continue to pass, ensuring no breaking changes.

Fixes #96.

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[twogood]

@copilot do not add dependency on any json package. Use a simple plain text body instead. Even if that is not the most common use case it is good enough for unit tests.

[Copilot]

@copilot do not add dependency on any json package. Use a simple plain text body instead. Even if that is not the most common use case it is good enough for unit tests.

Done! Removed the JSON package dependency and updated the tests to use plain text bodies instead. The tests are now much simpler and still effectively verify that CancellationToken parameters are not being used as request bodies. All tests continue to pass. (87f48ba)