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/src/webdriver_session.rs

@@ -120,8 +120,6 @@ impl WebDriverSession {
 
 #[cfg(test)]
 mod tests {
-    use super::*;
-
     #[test]
     fn test_webdriver_session_enum_variants() {
         // This test just verifies the enum structure compiles correctly