Fix optional Content-Type headers being transformed to enums (emitter-only)

[Copilot]

Optional Content-Type headers were incorrectly transformed into extensible enums by the type converter. They must remain as constants.

Changes

Emitter (type-converter.ts)

• Skip enum transformation for Content-Type headers, preserving constant type regardless of optionality

Emitter (operation-converter.ts)

• Pass header parameter to fromSdkType to enable Content-Type detection

Emitter Tests (operation-converter.test.ts)

• Added tests to verify Content-Type remains as constant type for both optional and required bodies

Result

Content-Type headers now remain as constants in the type model instead of being transformed to enums when the body parameter is optional. This prevents the enum transformation while preserving the constant type information for downstream processing.

Note

This PR focuses solely on the emitter-side fix to prevent unwanted enum transformation. Generator-side handling of optional Content-Type parameters (if needed for conditional setting) will be addressed separately.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Fix optional contentType issue</issue_title>
<issue_description>As described in Azure/azure-sdk-for-net#55300, but the issue belongs in this repo.</issue_description>

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

• Fixes Fix optional contentType issue #9494

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