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/client.rs

@@ -58,6 +58,7 @@ pub struct TdClient {
     pub network_state: NetworkState,
 }
 
+#[allow(dead_code)]
 impl TdClient {
     /// Creates a new TDLib client instance.
     ///