improve support for xml in accept

[eviltester]

I want Thingifier to recognise common XML-compatible media types,
so that API Challenges can demonstrate realistic XML content negotiation beyond only application/xml.

Background

Thingifier currently supports XML in a basic way using application/xml. It should expand XML support in the same spirit as JSON suffix support, so APIs can respond to generic XML, legacy XML, and vendor-specific XML media types where appropriate.

This should work with existing Accept header handling and should also be compatible with future q value negotiation.

Required Behaviour

Thingifier should treat these media types as valid XML requests for normal resource representations:

Accept: application/xml
Accept: text/xml
Accept: application/*+xml
Accept: application/vnd.apichallenges.todo+xml

For request bodies, Thingifier should also parse XML payloads when the request has:

Content-Type: application/xml
Content-Type: text/xml
Content-Type: application/vnd.apichallenges.todo+xml

Response Selection Rules

• application/xml should return the existing XML representation.
• text/xml should return the existing XML representation, with Content-Type: text/xml if the server chooses to echo the negotiated type.
• application/*+xml should match supported structured XML media types, such as application/vnd.apichallenges.todo+xml.
• application/vnd.apichallenges.todo+xml should return the normal todo XML representation, but with the vendor media type as the response Content-Type.
• Unsupported XML-based media types should not automatically be treated as todo XML.

Do Not Treat These As Generic Todo XML

These are XML-based, but semantically different. They should only work if explicitly implemented for that purpose:

application/problem+xml
application/soap+xml
application/xhtml+xml
image/svg+xml
application/atom+xml
application/rss+xml

For example:

Accept: application/problem+xml

Should not return a normal todo list as XML. It should return 406 Not Acceptable unless the endpoint is intentionally returning a problem-details error response.

Examples

Accept: application/xml

Returns todo XML.

Accept: text/xml

Returns todo XML.

Accept: application/vnd.apichallenges.todo+xml

Returns todo XML using the vendor XML media type.

Accept: application/*+xml

Returns a supported +xml representation if one exists.

Accept: application/problem+xml

Does not return the normal todo XML representation.

Content-Type: application/vnd.apichallenges.todo+xml

Allows XML request body parsing for create/update endpoints if the body matches the normal todo XML shape.

Acceptance Criteria

• Existing application/xml behaviour still works.
• text/xml is accepted for XML response negotiation.
• Vendor XML media types ending in +xml can be supported explicitly.
• application/*+xml can match supported XML vendor types.
• XML request bodies can be parsed from application/xml, text/xml, and supported vendor +xml content types.
• application/problem+xml, application/soap+xml, application/xhtml+xml, image/svg+xml, application/atom+xml, and application/rss+xml are not treated as normal todo XML by default.
• Tests cover Accept and Content-Type behaviour for generic XML, text XML, vendor XML, wildcard +xml, and unsupported XML-based media types.