proxy: replace http.Error with httputil.WriteErrorResponse for consistent JSON error shape

[Copilot]

internal/proxy/handler.go mixed http.Error (plain-text text/plain responses) with httputil.WriteErrorResponse/WriteJSONResponse (JSON), breaking client-side error handling. The httputil package explicitly documents that both server and proxy packages must use these helpers so API consumers always receive the same {"error":"...","message":"..."} shape.

Changes

• internal/proxy/handler.go: Replace all 7 http.Error calls with httputil.WriteErrorResponse, assigning appropriate error codes:

  HTTP status	error code
  400 Bad Request	bad_request
  403 Forbidden	forbidden
  502 Bad Gateway	bad_gateway
  503 Service Unavailable	service_unavailable
  500 Internal Server Error	internal_error

// Before — plain text, inconsistent Content-Type
http.Error(w, "upstream request failed", http.StatusBadGateway)

// After — uniform JSON shape across all error paths
httputil.WriteErrorResponse(w, http.StatusBadGateway, "bad_gateway", "upstream request failed")

[Copilot]

Pull request overview

This PR standardizes error responses emitted by the proxy handler so clients consistently receive JSON error bodies (via internal/httputil.WriteErrorResponse) instead of a mix of JSON and text/plain from http.Error, aligning proxy error behavior with the rest of the gateway.

Changes:

• Replaced 7 http.Error call sites in internal/proxy/handler.go with httputil.WriteErrorResponse.
• Normalized status-to-error-code mappings for common proxy failure paths (400/403/500/502/503).

Show a summary per file

File	Description
internal/proxy/handler.go	Switches all remaining plain-text http.Error responses to httputil.WriteErrorResponse to ensure a consistent JSON error shape.

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• Files reviewed: 1/1 changed files
• Comments generated: 1

[lpcox]

@copilot address review feedback

[Copilot]

@copilot address review feedback

Addressed in 7fbe817. I added regression assertions in internal/proxy/handler_test.go to verify the unknown REST endpoint and upstream network error paths return application/json with the expected {"error","message"} body.