Intelligently set content-type from body

[ryanfowler]

No description provided.

[Copilot]

Pull request overview

This PR refactors content-type handling to intelligently detect and set the content type from request body data, consolidating JSON and XML handling into a unified data field with automatic content type detection.

Changes:

• Replaced separate JSON and XML fields with a single Data field and ContentType field
• Moved content type detection logic from multipart package to a new shared core.DetectContentType function
• Updated CLI flags to set appropriate content types while using the unified Data field

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
main.go	Updated to pass ContentType field instead of separate JSON and XML fields to the request
internal/multipart/multipart.go	Refactored to use shared core.DetectContentType function, removing duplicate content type detection code
internal/fetch/fetch.go	Updated Request struct to use ContentType field instead of separate JSON and XML fields
internal/core/content_type.go	New file implementing shared content type detection logic with extension-based and byte-sniffing fallback
internal/client/client.go	Simplified request construction to use unified ContentType field instead of separate JSON/XML handling
internal/cli/app.go	Refactored CLI to set ContentType appropriately for --data, --json, and --xml flags using tracking booleans

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