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/streaming_parser_test.rs

@@ -75,7 +75,7 @@ fn test_first_complete_second_incomplete() {
     let content = r#"{"tool": "read_file", "args": {"file_path": "a.txt"}}
 {"tool": "shell", "args": {"command": "ls"#;
     
-    let tools = parser.process_chunk(&chunk(content, false));
+    let _tools = parser.process_chunk(&chunk(content, false));
     
     // Should detect the first complete tool call
     // The incomplete one should be detected by has_incomplete_tool_call