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-mcp-llm-server/Cargo.toml

@@ -12,9 +12,6 @@ serde_json = "1.0"
 anyhow = "1.0"
 tokio-stream = "0.1"
 
-[lib]
-path = "src/lib.rs"
-
 [[bin]]
 name = "owlen-mcp-llm-server"
-path = "src/lib.rs"
+path = "src/main.rs"