feat(core): 14 个公开枚举标记 #[non_exhaustive]

为 v0.2.0-rc.1 API 稳定性收尾。覆盖：
- P0 核心 IR: Message / ContentBlock / ContentBlockType / StreamEvent / HookEvent
- P0 Error: AgentError / LlmError / ToolError / MemoryError / PromptError
- P1 其他: MemoryStrategy / StepStatus / ToolChoice / ResponseFormat

明确不加的: 内部 wire-format (OpenaiChatMessage 等) /
语义已收敛 (Role/ServiceTier 等) / 使用面窄 (Permission 等)。

示例侧的 3 处 Message exhaustive match (prompt_composer /
conversation_memory_demo) 补全 `_` 通配分支，零行为变化。

验收: cargo build + clippy -D warnings + test --all-targets 全绿 (200 passed)

examples/conversation_memory_demo.rs

@@ -29,6 +29,7 @@ fn message_text(msg: &Message) -> &str {
             .next()
             .unwrap_or(""),
         Message::UserImage { .. } => "[image]",
+        _ => "",
     }
 }
 

examples/prompt_composer.rs

@@ -27,6 +27,7 @@ fn message_text(msg: &Message) -> String {
             })
             .collect(),
         Message::UserImage { .. } => "[image]".into(),
+        _ => String::new(),
     }
 }
 
@@ -90,6 +91,7 @@ fn main() {
             Message::User { .. } | Message::UserImage { .. } => "user",
             Message::Assistant { .. } => "assistant",
             Message::ToolResult { .. } => "tool",
+            _ => "unknown",
         };
         println!("[{i}] {role}: {}", message_text(m));
     }