Fix compiler warnings in test files

Eliminate unused variable and import warnings across test files:
- streaming_parser_test.rs: prefix unused `tools` with underscore
- webdriver_session.rs: remove unused `use super::*` import
- mock_provider_integration_test.rs: prefix unused `result` and `task_result`
- test_preflight_max_tokens.rs: prefix unused `proposed_max`
- todo_staleness_test.rs: add #[allow(dead_code)] for test helper methods
- json_parsing_stress_test.rs: prefix unused `tools`
- read_file_token_limit_test.rs: add #[allow(dead_code)] for unused helper
- background_process_demo_test.rs: remove unused PathBuf import
- test_session_continuation.rs: prefix unused `temp_dir` in 7 tests

All tests pass. No behavior changes.

Agent: fowler

crates/g3-core/tests/todo_staleness_test.rs

@@ -22,14 +22,17 @@ impl MockUiWriter {
         }
     }
 
+    #[allow(dead_code)]
     fn set_prompt_response(&self, response: bool) {
         self.prompt_responses.lock().unwrap().push(response);
     }
 
+    #[allow(dead_code)]
     fn set_choice_response(&self, response: usize) {
         self.choice_responses.lock().unwrap().push(response);
     }
 
+    #[allow(dead_code)]
     fn get_output(&self) -> Vec<String> {
         self.output.lock().unwrap().clone()
     }