feat(chat): implement AiChatRepository for SSE streaming

app/src/main/java/com/stand/standapp/ui/chat/repo/AiChatRepository.kt

@@ -0,0 +1,55 @@
+package com.stand.standapp.ui.chat.repo
+
+import kotlinx.coroutines.channels.awaitClose
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.callbackFlow
+import okhttp3.MediaType.Companion.toMediaType
+import okhttp3.OkHttpClient
+import okhttp3.Request
+import okhttp3.RequestBody.Companion.toRequestBody
+import okhttp3.Response
+import okhttp3.sse.EventSource
+import okhttp3.sse.EventSourceListener
+import okhttp3.sse.EventSources
+import timber.log.Timber
+import java.util.concurrent.TimeUnit
+
+class AiChatRepository {
+    // 根据后端实际情况修改，模拟器访问本地后端用 10.0.2.2
+    private val backendUrl = "http://10.0.2.2:8080/api/chat/stream" 
+    
+    private val client = OkHttpClient.Builder()
+        .readTimeout(0, TimeUnit.MILLISECONDS) // SSE 需要关闭读取超时
+        .build()
+
+    fun streamChat(message: String): Flow<String> = callbackFlow {
+        val json = """{"message": "$message"}"""
+        val request = Request.Builder()
+            .url(backendUrl)
+            .post(json.toRequestBody("application/json".toMediaType()))
+            .header("Accept", "text/event-stream")
+            .build()
+
+        val listener = object : EventSourceListener() {
+            override fun onEvent(eventSource: EventSource, id: String?, type: String?, data: String) {
+                // 收到后端推送的数据块
+                trySend(data)
+            }
+
+            override fun onFailure(eventSource: EventSource, t: Throwable?, response: Response?) {
+                Timber.e(t, "SSE Connection failed")
+                close(t) // 结束流
+            }
+
+            override fun onClosed(eventSource: EventSource) {
+                close() // 正常结束
+            }
+        }
+
+        val eventSource = EventSources.createFactory(client).newEventSource(request, listener)
+        
+        awaitClose {
+            eventSource.cancel()
+        }
+    }
+}