Fix Content-Type header set when request content is optional

[Copilot]

Fix Content-Type header setting in RestClient ✅

Summary

Fixed an issue where Content-Type headers were set unconditionally, even when request content was optional and might be null.

Changes Made

• Modified RestClientProvider.BuildCreateRequestMethodBody to detect and pass content parameter to AppendHeaderParameters
• Updated AppendHeaderParameters to wrap Content-Type header setting in if (content != null) check only when body parameter is optional
• Added comprehensive tests to verify the fix with full statement assertion
• Added test for required body parameter with assertion that Content-Type is NOT wrapped in if statement
• Removed unnecessary cast - ParameterProvider has implicit conversion to ValueExpression
• All 51 RestClientProvider tests passing

Result

Before:

// When body is optional
request.Headers.Set("Content-Type", "application/json");
request.Content = content;  // content could be null

After (when body is optional):

if (content != null)
{
    request.Headers.Set("Content-Type", "application/json");
}
request.Content = content;

After (when body is required):

request.Headers.Set("Content-Type", "application/json");
request.Content = content;  // content is required, no null check needed

This ensures Content-Type headers are only wrapped in null checks when the request body parameter is actually optional, avoiding unnecessary checks for required parameters.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Content-Type setting in RestClient is not correct</issue_title>
<issue_description>Currently we have code generated like this Azure/azure-sdk-for-net#55347 (comment). This isn't correct (obviously). We should instead be checking if the RequestContent is null when setting the Content-Type header.
</issue_description>

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

• Fixes Content-Type setting in RestClient is not correct #9520

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