fix: resolve all 40 clippy warnings (dead_code, unused_imports, lints)

- Add #[allow(unused_imports)] on pub re-exports used only by lib/tests
- Add #[allow(dead_code)] on public API items unused in binary target
- Fix collapsible_if, redundant_closure, unnecessary_map_or in main.rs
- Prefix unused test variables with underscore

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/tdlib/trait.rs

@@ -14,6 +14,7 @@ use super::ChatInfo;
 ///
 /// This trait defines the interface for both real and fake TDLib clients,
 /// enabling dependency injection and easier testing.
+#[allow(dead_code)]
 #[async_trait]
 pub trait TdClientTrait: Send {
     // ============ Auth methods ============