fix Content-Type header not being set for error responses

[paskal]

Summary

• Replace WriteHeader + RenderJSON pattern with EncodeJSON to ensure Content-Type: application/json header is set before WriteHeader is called
• Fixes Bug: WriteHeader before RenderJSON causes Content-Type header to be ignored rest#38 where responses had text/plain content-type instead of application/json

Affected endpoints:

• /auth/logout (when no providers defined) → 400 Bad Request
• /auth/user (when not authenticated) → 401 Unauthorized
• Auth endpoints with unsupported provider → 400 Bad Request

[coveralls]

Pull Request Test Coverage Report for Build 20277117952

Details

• 12 of 12 (100.0%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 83.652%

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Totals
Change from base Build 19937627146:	-0.02%
Covered Lines:	2671
Relevant Lines:	3193

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💛 - Coveralls