fix(user_tools): download explicit tools jar

[WilliamK112]

Summary

• download explicitly supplied --tools_jar values even when the packaged tools jar is available locally
• keep the direct local-resource fast path only for the default wheel-provided jar
• add unit coverage for the default local-resource jar path and an explicit remote gs:// tools jar

Root cause

When use_local_tools_jar() is true, _process_jar_arg() used the jar path directly without checking whether the user had supplied --tools_jar. That lets a remote jar URL flow into the Java classpath unchanged, which causes Java to fail loading the tools main class.

Fixes #1889

Validation

• .venv/bin/python -m pytest tests/spark_rapids_tools_ut/test_path_boundary.py -q
• .venv/bin/python -m flake8 tests/spark_rapids_tools_ut/test_path_boundary.py src/spark_rapids_pytools/rapids/rapids_tool.py
• .venv/bin/python -m pylint -d fixme --load-plugins pylint_pydantic --rcfile=../.pylintrc tests/spark_rapids_tools_ut/test_path_boundary.py src/spark_rapids_pytools/rapids/rapids_tool.py
• git diff --check

Note: test_tool_argprocessor.py was also attempted locally, but the local macOS environment is missing libomp.dylib, so unrelated XGBoost imports fail before these changes are exercised.