[http-client-csharp] Place contentType after body per .NET SDK parameter ordering guideline

[Copilot]

Operations with a contentType header placed it before body, violating the .NET SDK parameter ordering guideline. This applies to all operations (both multipart and non-multipart).

Before:

UpdateSkillDefaultVersion(string skillId, string contentType, BinaryContent content, RequestOptions options = null)

After:

UpdateSkillDefaultVersion(string skillId, BinaryContent content, string contentType, RequestOptions options = null)

Changes

• RestClientProvider.GetMethodParameters: Added a contentType sorting bucket (350) between body (200/300) and optional query/header (400). Content-type headers detected via InputHeaderParameter.IsContentType are routed to this bucket instead of the regular required/optional header buckets. Updated the IsMultipartFormData content-type insertion to also use the contentType bucket for consistency.
• Back-compat via BackCompatProvider: Added HasContentTypeBeforeBodyInLastContract helper that checks client.BackCompatProvider.LastContractView for a matching method where contentType appears before content (body). If the previous contract had that ordering, the content-type parameter falls through to the regular header bucket to preserve backward compatibility.
• Tests: Added parameterized TestGetMethodParameters_ContentTypeAfterBody(bool isRequired, bool isExtensibleEnum) using [TestCase] to cover all four combinations of required/optional and string/extensible-enum content-type headers. Added ContentTypeOrderPreservedFromLastContractView validating that the previous ordering is preserved when the last contract had contentType before body, and ContentTypeAfterBodyInLastContractView validating correct ordering when the last contract already had contentType after body.
• Documentation: Updated backward-compatibility.md with a new "Content-Type Parameter Ordering" section documenting the back-compat preservation case.

[jorgerangel-msft]

@copilot we should also ensure we run Generate.ps1 to regenerate all the test libraries

[Copilot]

@copilot we should also ensure we run Generate.ps1 to regenerate all the test libraries

Ran eng/scripts/Generate.ps1 — it completed successfully with no file changes. The generated test libraries are already up to date since none of the Spector specs or Sample-TypeSpec include operations with union content-type headers that would be affected by the parameter reordering. All 1305 unit tests continue to pass.

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/http-client-csharp@10403

commit: 036e36c

[Copilot]

@copilot we should also ensure we run Generate.ps1 to regenerate all the test libraries

Ran eng/scripts/Generate.ps1 after the latest changes — completed successfully with no generated file changes. All 1306 tests pass.