Unify integration test environment and add valid config validation

- Create shared tests/common.rs with TestEnvironment setup
- Simplify gemini_test_client.py to single-request client
- Refactor config validation tests to use common setup
- Add test_valid_config_startup for complete server validation
- Fix clippy warning in main.rs
- Remove unused code and consolidate test infrastructure

src/main.rs

@@ -61,7 +61,7 @@ async fn main() {
     }
 
     // Load and parse config
-    let config = match config::load_config(&config_path) {
+    let config = match config::load_config(config_path) {
         Ok(config) => config,
         Err(e) => {
             eprintln!("Error: Failed to parse config file '{}': {}", config_path, e);