fix(openai): recognize MiMo in thinking disable

openAIThinkingDisableKind only matched dashscope/qwen/aliyun,
deepseek/bigmodel/z.ai/zhipu, and gpt-5.1+/gpt-6. Xiaomi MiMo
(api.xiaomimimo.com, model mimo-*) fell through to the default
branch, so when a user set reasoning=disabled the adapter wrote
no disable field and MiMo kept thinking enabled (extra latency +
reasoning token cost contrary to user intent).

Add xiaomimimo/mimo to the thinking_type case — MiMo uses the same
thinking:{type:"disabled"} shape as deepseek/glm, confirmed in
MiMo docs.

- openai.go: +2 conditions in openAIThinkingDisableKind
- openai_thinking_disable_test.go: cover MiMo (base/model/ultraspeed),
  regress deepseek/glm/z.ai/qwen/gpt-5.1/gpt-6/unknown, and verify
  applyOpenAIThinkingDisable writes thinking:{type:disabled} +
  deletes reasoning_effort for MiMo

internal/backend/agent/model/openai.go

@@ -1893,10 +1893,13 @@ func openAIThinkingDisableKind(baseURL string, modelID string, endpoint string)
 		strings.Contains(base, "bigmodel") ||
 		strings.Contains(base, "z.ai") ||
 		strings.Contains(base, "zhipu") ||
+		strings.Contains(base, "xiaomimimo") ||
+		strings.Contains(base, "mimo") ||
 		strings.Contains(model, "deepseek") ||
 		strings.Contains(model, "glm") ||
 		strings.Contains(model, "zai") ||
-		strings.Contains(model, "zhipu"):
+		strings.Contains(model, "zhipu") ||
+		strings.Contains(model, "mimo"):
 		return "thinking_type"
 	case openAIModelSupportsReasoningNone(model):
 		return "reasoning_none"