This PR fixes content-type negotiation when clients request text/plain responses.

/claim #3284

Changes:

• Modified content negotiation to properly respect Accept headers
• Ensured consistent behavior between success and error responses
• Added test cases for content negotiation scenarios

Fixes:

• #3284 (Content-Type header incorrect)
• #3257 (Error codec media type handling)

Testing:

• Verified text/plain responses when requested
• Confirmed JSON remains default when no Accept header
• Verified error responses respect content type