feat: add proxy integration tests and fix Codex↔Gemini response converter bug (#393)

Add comprehensive end-to-end proxy integration tests covering all 4 protocol
types (Claude, OpenAI, Codex, Gemini) with 20 test cases including passthrough,
all 12 cross-protocol conversions, error handling, and multi-protocol coexistence.

Fix a bug where Codex↔Gemini response transformers were registered with swapped
implementations, causing silent data corruption: responses would be returned in
the wrong format with null/zero fields instead of being properly converted.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: Bowl42

internal/converter/codex_to_gemini.go

@@ -9,7 +9,9 @@ import (
 )
 
 func init() {
-	RegisterConverter(domain.ClientTypeCodex, domain.ClientTypeGemini, &codexToGeminiRequest{}, &codexToGeminiResponse{})
+	// Response transformer is geminiToCodexResponse (reads Codex body → outputs Gemini body)
+	// because responses[from][to] must convert FROM from-format TO to-format
+	RegisterConverter(domain.ClientTypeCodex, domain.ClientTypeGemini, &codexToGeminiRequest{}, &geminiToCodexResponse{})
 }
 
 type codexToGeminiRequest struct{}

internal/converter/gemini_to_codex.go

@@ -9,7 +9,9 @@ import (
 )
 
 func init() {
-	RegisterConverter(domain.ClientTypeGemini, domain.ClientTypeCodex, &geminiToCodexRequest{}, &geminiToCodexResponse{})
+	// Response transformer is codexToGeminiResponse (reads Gemini body → outputs Codex body)
+	// because responses[from][to] must convert FROM from-format TO to-format
+	RegisterConverter(domain.ClientTypeGemini, domain.ClientTypeCodex, &geminiToCodexRequest{}, &codexToGeminiResponse{})
 }
 
 type geminiToCodexRequest struct{}