fix: 修复 Agent 流式对话无响应和工具 schema 兼容性问题

- 在 `run_stream()` LLM 调用前 yield `think` 事件，前端即时显示"思考中..."
- 修复 tool schema 规范化逻辑：`{"function":{...}}` 格式缺少 `type` 字段导致 LLM API 拒绝
- 启动时从数据库加载自定义工具（`load_tools_from_db`），解决重启后工具丢失
- 前端 SSE 添加 60s 超时保护，任何事件类型均触发 `receivedFirstEvent`
- 流式失败自动降级到非流式 POST
- 添加 `scripts/seed_coding_agent.py` 和 `scripts/test_coding_agent.py`

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/views/AgentChat.vue

@@ -396,9 +396,11 @@ async function sendMessage() {
         ? `/api/v1/agent-chat/${currentAgentId.value}/stream`
         : '/api/v1/agent-chat/bare/stream'
 
-      // 尝试 SSE 流式
+      // 尝试 SSE 流式（带超时控制）
       let usedStreaming = false
       streamingActive.value = false
+      const abortController = new AbortController()
+      const streamTimeout = setTimeout(() => abortController.abort(), 60000)
       try {
         const token = localStorage.getItem('token') || ''
         const resp = await fetch(streamEndpoint, {
@@ -408,6 +410,7 @@ async function sendMessage() {
             ...(token ? { 'Authorization': `Bearer ${token}` } : {}),
           },
           body: JSON.stringify({ message: text, session_id: sessId || undefined }),
+          signal: abortController.signal,
         })
 
         if (resp.ok && resp.body) {
@@ -447,16 +450,17 @@ async function sendMessage() {
               try {
                 const data = JSON.parse(dataStr)
 
-                // 首个事件到达 → 隐藏 loading dots
-                if (!receivedFirstEvent && (eventType === 'think' || eventType === 'tool_call' || eventType === 'tool_result')) {
+                // 首个事件到达 → 隐藏 loading dots，无论什么事件类型
+                if (!receivedFirstEvent) {
                   receivedFirstEvent = true
                   streamingActive.value = true
                 }
 
                 if (eventType === 'think') {
+                  const thinkContent = data.content || '思考中...'
                   currentMsg.steps!.push({
                     iteration: data.iteration, type: 'think',
-                    content: data.content || '',
+                    content: thinkContent,
                     reasoning: data.reasoning,
                     tool_name: data.tool_names?.[0],
                   })
@@ -491,7 +495,10 @@ async function sendMessage() {
             }
           }
         }
-      } catch { /* 流式不可用，降级到普通 POST */
+      } catch {
+        clearTimeout(streamTimeout)
+        // 流式失败时标记为非流式，让 fallback POST 兜底
+        usedStreaming = false
         streamingActive.value = false
       }