Implement Content Negotiation to Support Multiple Response Formats

To accommodate diverse client preferences, I propose introducing content negotiation. This feature allows clients to request their preferred content format (e.g., JSON, XML, HTML) via the Accept header, enabling the API to automatically adapt and serve the optimal media type.

The issue names no files or tests; start by reviewing the repository’s OpenAPI response-generation paths for Hono and itty-router. Define how Accept headers select JSON, XML, or HTML responses, and consider the work complete when the supported formats and selection behavior are specified and covered.

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