improve json accept header processing

[eviltester]

I want Thingifier to parse and apply Accept headers correctly for JSON, XML, wildcards, quality values, and structured +json media types, so API responses use a supported representation and reject unsupported JSON-shaped media types such as application/problem+json.

Background
Thingifier currently supports standard API representations such as application/json and application/xml. Some Accept headers can still be JSON-related without being application/json, e.g. application/*+json, application/problem+json, or vendor types like application/vnd.api+json. These should not automatically be treated as supported unless Thingifier has an explicit representation writer for them.

Required Behavior

• Accept: application/json returns JSON with Content-Type: application/json.
• Missing Accept or Accept: */* returns the existing default representation, normally application/json.
• Accept: application/xml returns XML when the endpoint supports XML.
• Accept headers with multiple values must respect q values.
• q=0 means “not acceptable” and must not be selected.
• Unsupported media types must be ignored when a supported alternative is present.
• If no acceptable supported representation exists, return 406 Not Acceptable.

Structured JSON Rules

• Do not treat every +json media type as application/json.
• application/problem+json, application/vnd.api+json, application/hal+json, and similar types should return 406 Not Acceptable unless Thingifier explicitly supports that exact representation.
• Accept: application/*+json means the client accepts structured +json media types. It should not match plain application/json.
• If Thingifier later adds a concrete +json representation, e.g. application/problem+json, then application/*+json may match that supported concrete type.

Examples

• Accept: application/json, application/problem+json returns application/json.
• Accept: application/problem+json, application/json;q=0.5 returns application/json.
• Accept: application/problem+json returns 406.
• Accept: application/*+json returns 406 unless a supported +json representation exists.
• Accept: application/json;q=0, application/xml returns XML.
• Accept: application/json;q=0 returns 406.

Acceptance Criteria

• Add an Accept header parser that handles comma-separated media ranges, parameters, and q values.
• Add content negotiation tests for exact matches, wildcards, unsupported media types, +json suffix types, and q=0.
• Keep existing default JSON behavior for no Accept and */*.
• Ensure application/problem+json is rejected until explicitly implemented.
• Ensure chosen response Content-Type matches a representation Thingifier can actually serialize.