fix: keep builder file operations under project root

[maxts0gt]

Summary

• resolve Agent Builder file paths against the session project root
• reject absolute, parent-directory, and symlink paths that escape that root
• add regression coverage for read/write/delete file tools refusing outside paths

Tests

• uv run --with pytest --with pytest-asyncio pytest tests/unittests/cli/built_in_agents/test_builder_file_paths.py
• uv run --with pyink pyink --check src/google/adk/cli/built_in_agents/utils/resolve_root_directory.py tests/unittests/cli/built_in_agents/test_builder_file_paths.py