refactor(model): 拆 openai.go 652→216行(纯搬移零逻辑改动) — encode/response/stream 三件套对齐 anthropic 布局

- openai.go 216(适配器+Chat/ChatStream+共享助手); openai_encode.go 202(请求wire+build*); openai_response.go 79(响应wire+responseFromWire); openai_stream.go 174(scanSSE)
- 字节级比对 body identical; 导出API不变(go doc diff 空); gofmt 干净
- 纯净基线(HEAD+本改)全仓 go build/vet/test 绿 + win/darwin 交叉编译绿
- 遗留: scanSSE 138行(>80, 拆分前既有,零逻辑改动未动), 另开步骤重构

internal/runtime/model/openai_response.go

@@ -0,0 +1,79 @@
+package model
+
+import (
+	"encoding/json"
+)
+
+// openAIChatResponse is the JSON body returned by /chat/completions, for both
+// the non-streaming payload and each SSE stream chunk.
+type openAIChatResponse struct {
+	ID      string         `json:"id"`
+	Model   string         `json:"model"`
+	Choices []openAIChoice `json:"choices"`
+	Usage   *openAIUsage   `json:"usage,omitempty"`
+}
+
+// openAIChoice is one candidate completion.
+type openAIChoice struct {
+	Index        int                   `json:"index"`
+	Message      openAIResponseMessage `json:"message"`
+	Delta        *openAIDelta          `json:"delta,omitempty"`
+	FinishReason string                `json:"finish_reason,omitempty"`
+}
+
+// openAIResponseMessage is the assistant message inside a non-streaming choice.
+type openAIResponseMessage struct {
+	Role      string           `json:"role"`
+	Content   string           `json:"content"`
+	ToolCalls []openAIToolCall `json:"tool_calls,omitempty"`
+}
+
+// openAIDelta is the incremental payload inside a stream chunk. Reasoning content
+// (OpenAI o-series / DeepSeek / DashScope) arrives in ReasoningContent; streaming
+// function calls arrive in ToolCalls, accumulated per fragment index.
+type openAIDelta struct {
+	Role             string                `json:"role"`
+	Content          string                `json:"content"`
+	ReasoningContent string                `json:"reasoning_content"`
+	ToolCalls        []openAIToolCallDelta `json:"tool_calls"`
+}
+
+// openAIToolCallDelta is one streaming function-call fragment. OpenAI emits a
+// sequence of these sharing the same Index: the ID and Function.Name usually
+// arrive once (in the first fragment) while Function.Arguments streams across
+// fragments.
+type openAIToolCallDelta struct {
+	Index    int              `json:"index"`
+	ID       string           `json:"id"`
+	Function openAIToolCallFn `json:"function"`
+}
+
+// openAIUsage reports token consumption.
+type openAIUsage struct {
+	PromptTokens     int `json:"prompt_tokens"`
+	CompletionTokens int `json:"completion_tokens"`
+	TotalTokens      int `json:"total_tokens"`
+}
+
+func responseFromWire(wire *openAIChatResponse) Response {
+	out := Response{Model: wire.Model}
+	if len(wire.Choices) > 0 {
+		text := wire.Choices[0].Message.Content
+		out.Content = text
+		out.Message = Message{Role: RoleAssistant, Content: []Content{{Type: ContentTypeText, Text: text}}}
+		out.StopReason = wire.Choices[0].FinishReason
+		for _, tc := range wire.Choices[0].Message.ToolCalls {
+			args := map[string]any{}
+			if tc.Function.Arguments != "" {
+				_ = json.Unmarshal([]byte(tc.Function.Arguments), &args)
+			}
+			out.ToolCalls = append(out.ToolCalls, ToolCall{Name: tc.Function.Name, ID: tc.ID, Args: args})
+		}
+	}
+	if u := wire.Usage; u != nil {
+		out.PromptTokens = u.PromptTokens
+		out.CompletionTokens = u.CompletionTokens
+		out.TotalTokens = u.TotalTokens
+	}
+	return out
+}