模型支持协议扩展参数，用于关闭思考模式

DeepSeek 现役的 v4 系列全部默认开启思考模式，没有单独的非思考型模型
可换；官方给的关闭方式是在请求里带 thinking={"type":"disabled"}。

ai_models 表早有 extra_json 字段但从未接线，这里接通：模型配置里的
JSON 对象会合并进每次调用的请求体，因此任何协议级参数都能在管理端
配置，不必改代码。合并不会覆盖 model/messages/max_tokens/temperature/
stream —— 预算和结构由代码掌握，重试逻辑才不会被配置绕过。
写入前校验 JSON，避免非法值存进 JSON 列后每次调用才报错。

线上模型已配置为关闭思考：同一会话下由 10133 ms / 1024 输出 tokens、
回复被截断，变为 669 ms / 10 tokens、回复完整。max_tokens 相应回调至 512。

管理端模型表单新增「协议参数」输入框（该目录尚未纳入版本库，仅已发布）。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

im/backend/internal/app/ai_providers_test.go

@@ -320,3 +320,51 @@ func TestTruncatedAnswerIsRetriedAndKeptIfItSurvives(t *testing.T) {
 		t.Fatalf("text = %q", result.Text)
 	}
 }
+
+// A model that reasons by default is told not to through its stored protocol
+// fields, without a code change and without a second model row.
+func TestModelExtraFieldsReachTheProviderWithoutOverridingTheCall(t *testing.T) {
+	server, body, _ := captureAIRequest(t, `{"id":"c","choices":[{"message":{"content":"在的"},"finish_reason":"stop"}],"usage":{}}`, 200)
+	model := testModel("openai", server.URL)
+	model.Extra = map[string]any{
+		"thinking": map[string]any{"type": "disabled"},
+		// Structural fields and the budget stay under this code's control.
+		"model":      "someone-elses-model",
+		"max_tokens": 999999,
+		"stream":     true,
+	}
+	provider, err := newAIProvider(model, false)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err = provider.Chat(context.Background(), aiChatRequest{Messages: []aiChatMessage{{Role: "user", Text: "在吗"}}}); err != nil {
+		t.Fatal(err)
+	}
+	thinking, ok := (*body)["thinking"].(map[string]any)
+	if !ok || thinking["type"] != "disabled" {
+		t.Fatalf("thinking = %v", (*body)["thinking"])
+	}
+	if (*body)["model"] != "demo-model" || (*body)["max_tokens"] != float64(128) || (*body)["stream"] != false {
+		t.Fatalf("扩展参数不得覆盖调用本身: %v", *body)
+	}
+}
+
+func TestAIModelExtraIsValidatedBeforeItIsStored(t *testing.T) {
+	keep := "{\"thinking\":{\"type\":\"disabled\"}}"
+	value, err := aiModelExtra(&keep, nil)
+	if err != nil || value != keep {
+		t.Fatalf("value = %v err = %v", value, err)
+	}
+	blank := "  "
+	if value, err = aiModelExtra(&blank, "old"); err != nil || value != nil {
+		t.Fatalf("留空应清除扩展参数: %v %v", value, err)
+	}
+	broken := "{not json"
+	if _, err = aiModelExtra(&broken, nil); err == nil {
+		t.Fatal("非法 JSON 必须在写库前被拒绝")
+	}
+	// Omitted entirely: the stored value survives an edit that does not mention it.
+	if value, err = aiModelExtra(nil, "old"); err != nil || value != "old" {
+		t.Fatalf("value = %v err = %v", value, err)
+	}
+}