fix: resolve all compilation errors and clippy warnings

This commit fixes 12 categories of errors across the codebase:

- Fix owlen-mcp-llm-server build target conflict by renaming lib.rs to main.rs
- Resolve ambiguous glob re-exports in owlen-core by using explicit exports
- Add Default derive to MockMcpClient and MockProvider test utilities
- Remove unused imports from owlen-core test files
- Fix needless borrows in test file arguments
- Improve Config initialization style in mode_tool_filter tests
- Make AgentExecutor::parse_response public for testing
- Remove non-existent max_tool_calls field from AgentConfig usage
- Fix AgentExecutor::new calls to use correct 3-argument signature
- Fix AgentResult field access in agent tests
- Use Debug formatting instead of Display for AgentResult
- Remove unnecessary default() calls on unit structs

All changes ensure the project compiles cleanly with:
- cargo check --all-targets ✓
- cargo clippy --all-targets -- -D warnings ✓
- cargo test --no-run ✓

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

crates/owlen-core/src/agent.rs

@@ -235,7 +235,7 @@ impl AgentExecutor {
     }
 
     /// Parse LLM response into structured format
-    fn parse_response(&self, text: &str) -> Result<LlmResponse> {
+    pub fn parse_response(&self, text: &str) -> Result<LlmResponse> {
         let lines: Vec<&str> = text.lines().collect();
         let mut thought = String::new();
         let mut action = String::new();
@@ -370,8 +370,8 @@ mod tests {
     #[test]
     fn test_parse_tool_call() {
         let executor = AgentExecutor {
-            llm_client: Arc::new(MockProvider::new()),
-            tool_client: Arc::new(MockMcpClient::new()),
+            llm_client: Arc::new(MockProvider),
+            tool_client: Arc::new(MockMcpClient),
             config: AgentConfig::default(),
         };
 
@@ -399,8 +399,8 @@ ACTION_INPUT: {"query": "Rust programming language"}
     #[test]
     fn test_parse_final_answer() {
         let executor = AgentExecutor {
-            llm_client: Arc::new(MockProvider::new()),
-            tool_client: Arc::new(MockMcpClient::new()),
+            llm_client: Arc::new(MockProvider),
+            tool_client: Arc::new(MockMcpClient),
             config: AgentConfig::default(),
         };