Add thinking duration to message parts

Why: The chat timeline needs a persisted thinking duration, and tool duration already lives on the tool call, so the thinking part had nowhere to put it.
Evidence: go test ./... passes; the message golden now includes durationMs on the thinking part.
Impact: Additive JSON field with omitempty. Existing clients ignore unknown fields.
Refs: feature/chat-timeline-preview
Co-authored-by: Cursor <cursoragent@cursor.com>

conformance_test.go

@@ -42,7 +42,7 @@ func sampleMessage() Message {
 		ID: "msg_abc123def456", ConversationID: "cnv_abc123def456", RunID: "run_abc123def456",
 		Role: RoleAssistant,
 		Parts: []Part{
-			{Type: PartThinking, Text: "考虑先检索…", Process: true},
+			{Type: PartThinking, Text: "考虑先检索…", Process: true, DurationMs: 1500},
 			{Type: PartToolCall, Process: true, ToolCall: &ToolCallPart{ID: "call_1", Name: "history_search", Arguments: `{"q":"部署"}`, Duration: 12.5}},
 			{Type: PartToolResult, Process: true, ToolResult: &ToolResultPart{CallID: "call_1", Status: "ok", Summary: "3 条命中"}},
 			{Type: PartText, Text: "根据历史记录……"},

model.go

@@ -89,6 +89,10 @@ type Part struct {
 
 	// 过程标记：true 的部件在 run 终态后由前端折叠为摘要行（4.2）。
 	Process bool `json:"process,omitempty"`
+
+	// DurationMs 是 thinking 部件的思考时长（毫秒，从首个推理增量到最后一个）。
+	// 工具耗时在 ToolCallPart.Duration，不复用本字段。
+	DurationMs float64 `json:"durationMs,omitempty"`
 }
 
 type ToolCallPart struct {

testdata/golden/message.json

@@ -7,7 +7,8 @@
     {
       "type": "thinking",
       "text": "考虑先检索…",
-      "process": true
+      "process": true,
+      "durationMs": 1500
     },
     {
       "type": "tool_call",

ts/types.ts

@@ -35,6 +35,7 @@ export interface Part {
   card?: { kind: string; data: UINode[]; cardId?: ID }; // cardId 非空 = 收件箱卡片引用部件（submit_card）
   error?: { code: string; message: string };
   process?: boolean; // run 终态后折叠为摘要行
+  durationMs?: number; // thinking 部件的思考时长（毫秒）；工具耗时在 toolCall.durationMs
 }
 
 export interface Message {