fix(ai): Agent chat handler 精确选择 FC-capable provider + 环境变量适配

- chat_handler: 使用 get_provider("claude") 精确获取，避免 resolve fallback 到 Ollama
- ProviderRegistry: 新增 get_provider() 方法（无 health check，无 fallback）
- orchestrator: 从 ANTHROPIC_DEFAULT_SONNET_MODEL 读取模型名，兼容智谱代理
- erp-server: Claude provider 注册优先读 ANTHROPIC_AUTH_TOKEN + ANTHROPIC_BASE_URL

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/erp-ai/src/handler/chat_handler.rs

@@ -128,14 +128,16 @@ where
         tool_call_id: None,
     });
 
-    // 解析 Provider
-    let resolved = ai_state
+    // 解析 Provider — Agent 需要 Function Calling，精确获取 Claude/OpenAI
+    let provider_arc = ai_state
         .provider_registry
-        .resolve("auto")
-        .await
-        .map_err(|e| {
-            tracing::error!(error = %e, "AI provider resolve failed");
-            erp_core::error::AppError::Internal("AI 服务暂时不可用，请稍后再试".into())
+        .get_provider("claude")
+        .or_else(|| ai_state.provider_registry.get_provider("openai"))
+        .ok_or_else(|| {
+            tracing::error!("No FC-capable provider found (need claude or openai)");
+            erp_core::error::AppError::Internal(
+                "AI Agent 暂时不可用，需要 Claude 或 OpenAI 提供商".into(),
+            )
         })?;
 
     // 构建 ToolRegistry — Phase 0 只有 query_patient_vitals
@@ -159,7 +161,6 @@ where
     );
 
     // 执行 Agent ReAct 循环
-    let provider_arc = resolved.into_arc();
     let orchestrator = AgentOrchestrator::new(provider_arc, std::sync::Arc::new(registry));
     let result = orchestrator
         .run(SYSTEM_PROMPT, &mut messages, &tool_ctx)