Content negotiation via Accept header

[bjaus]

Auto-select response format based on the Accept request header. At minimum support application/json and application/xml, with application/json as the default fallback.

Scope:

• Parse Accept header with quality values (e.g., application/xml;q=0.9, application/json;q=1.0)
• Route to the appropriate encoder
• Return 406 Not Acceptable if no supported format matches
• Integrate with the existing WithEncoder customization point

[bjaus]

Implemented in bc873c5 — codecRegistry negotiates Accept header, returns 406 for unsupported types, JSON default for empty// accept.