feat(runtime): add support for gpt-image-1.5 and direct image API proxying

- Introduced the `gpt-image-2` model in Codex built-ins and updated visibility logic in the registry.
- Added direct proxy support for OpenAI image generation and editing endpoints.
- Implemented new execution paths for `/images/generations` and `/images/edit`, ensuring seamless handling for both JSON and multipart payloads.
- Expanded test coverage to validate the new model and direct proxy features, including streaming scenarios and error handling.

Author: luispater

config.example.yaml

@@ -132,7 +132,7 @@ disable-claude-cloak-mode: false
 # - "passthrough": never inject or strip image_generation on non-images endpoints (forward the client payload unchanged); behaves like "chat" on /v1/images/* endpoints.
 disable-image-generation: false
 
-# Base model used when proxying gpt-image-2 via the hosted image_generation tool (Responses API).
+# Base model used by the legacy hosted image_generation tool path when a Codex image request is not proxied directly through the Image API.
 # Must start with "gpt-" (case-insensitive). If unset or invalid, defaults to "gpt-5.4-mini".
 # gpt-image-2-base-model: "gpt-5.4-mini"
 

internal/config/sdk_config.go

@@ -21,8 +21,9 @@ type SDKConfig struct {
 	//     sent it and do not inject it otherwise; on /v1/images/generations and /v1/images/edits behave like "chat".
 	DisableImageGeneration DisableImageGenerationMode `yaml:"disable-image-generation" json:"disable-image-generation"`
 
-	// GPTImage2BaseModel sets the base (mainline) model used when proxying GPT Image 2
-	// requests via the hosted image_generation tool (e.g. Codex OAuth /v1/images/*).
+	// GPTImage2BaseModel sets the base (mainline) model used by the legacy hosted
+	// image_generation tool path when a Codex image request is not proxied directly
+	// through the Image API.
 	//
 	// The value must start with "gpt-" (case-insensitive). If empty or invalid, the
 	// default base model ("gpt-5.4-mini") is used.

internal/registry/model_definitions.go

@@ -7,6 +7,7 @@ import (
 )
 
 const (
+	codexBuiltinImage15ModelID      = "gpt-image-1.5"
 	codexBuiltinImageModelID        = "gpt-image-2"
 	xaiBuiltinImageModelID          = "grok-imagine-image"
 	xaiBuiltinImageQualityModelID   = "grok-imagine-image-quality"
@@ -119,7 +120,7 @@ func GetXAIModels() []*ModelInfo {
 // not depend on remote models.json updates. Built-ins replace any matching IDs
 // already present in the provided slice.
 func WithCodexBuiltins(models []*ModelInfo) []*ModelInfo {
-	return upsertModelInfos(models, codexBuiltinImageModelInfo())
+	return upsertModelInfos(models, codexBuiltinImage15ModelInfo(), codexBuiltinImageModelInfo())
 }
 
 // WithXAIBuiltins injects hard-coded xAI image/video model definitions that should
@@ -136,6 +137,18 @@ func normalizeAntigravityCapabilityModelID(modelID string) string {
 	return modelID
 }
 
+func codexBuiltinImage15ModelInfo() *ModelInfo {
+	return &ModelInfo{
+		ID:          codexBuiltinImage15ModelID,
+		Object:      "model",
+		Created:     1704067200, // 2024-01-01
+		OwnedBy:     "openai",
+		Type:        "openai",
+		DisplayName: "GPT Image 1.5",
+		Version:     codexBuiltinImage15ModelID,
+	}
+}
+
 func codexBuiltinImageModelInfo() *ModelInfo {
 	return &ModelInfo{
 		ID:          codexBuiltinImageModelID,