feat(llm): 公开 MockProvider 并修复测试与 API 一致性

- 新增 src/llm/mock.rs：公开 MockProvider，实现 chat 与 chat_stream
  (chat_stream 从 MessageResponse 拆解为 StreamEvent 序列)，含 5 个内联测试
- src/llm.rs 注册 pub mod mock，供示例与外部 crate 引用
- src/agent/session.rs 测试改用 crate::llm::mock::MockProvider，
  删除内嵌私有副本，统一公开路径
- src/prompt.rs re-export validate_messages，与 PromptComposer 对齐公开路径

src/agent/session.rs

@@ -175,16 +175,12 @@ impl AgentSession {
 mod tests {
     use super::*;
     use crate::agent::builder::AgentBuilder;
-    use crate::llm::error::LlmError;
     use crate::llm::hooks::{Hook, HookContext, HookExecutor, HookResult};
-    use crate::llm::provider::{LlmProvider, ProviderCapabilities, ProviderFeatures};
+    use crate::llm::mock::MockProvider;
     use crate::llm::types::message::ContentBlock;
-    use crate::llm::types::request_v2::MessageRequest;
-    use crate::llm::types::response_v2::{MessageResponse, StopReason, StreamEvent};
+    use crate::llm::types::response_v2::{MessageResponse, StopReason};
     use crate::tools::ToolRegistry;
     use async_trait::async_trait;
-    use futures_core::Stream;
-    use std::pin::Pin;
     use std::sync::atomic::{AtomicU32, Ordering};
 
     /// 计数 hook —— 每被调用一次 +1。
@@ -208,44 +204,6 @@ mod tests {
         }
     }
 
-    /// MockProvider：按调用顺序返回预设响应。
-    struct MockProvider {
-        responses: std::sync::Mutex<Vec<MessageResponse>>,
-    }
-
-    impl MockProvider {
-        fn new(responses: Vec<MessageResponse>) -> Self {
-            Self {
-                responses: std::sync::Mutex::new(responses),
-            }
-        }
-    }
-
-    #[async_trait]
-    impl LlmProvider for MockProvider {
-        async fn chat(&self, _request: MessageRequest) -> Result<MessageResponse, LlmError> {
-            let mut responses = self.responses.lock().unwrap();
-            if responses.is_empty() {
-                return Err(LlmError::Other("no more mock responses".into()));
-            }
-            Ok(responses.remove(0))
-        }
-        async fn chat_stream(
-            &self,
-            _request: MessageRequest,
-        ) -> Result<Pin<Box<dyn Stream<Item = Result<StreamEvent, LlmError>> + Send>>, LlmError>
-        {
-            unimplemented!()
-        }
-        fn capabilities(&self) -> ProviderCapabilities {
-            ProviderCapabilities {
-                provider_name: "mock",
-                supported_models: None,
-                features: ProviderFeatures::default(),
-            }
-        }
-    }
-
     struct StubAgent {
         name: String,
         prompt: Option<String>,