[tontester] add optional dependency group for consensus explorer

[yungwine]

Usage: uv run --extra consensus_explorer ... or uv sync --extra consensus_explorer; uv run ...

[github-actions]

1. Potential runtime break for default install
   • test/tontester/pyproject.toml:16 still installs the consensus_explorer console script unconditionally, but its dependencies are now optional (test/tontester/pyproject.toml:9).
   • The entry module imports DashApp at import time (test/tontester/src/consensus_explorer/explorer.py:8), so running consensus_explorer without the extra will crash with ModuleNotFoundError (e.g., missing dash).
   • Suggestion: defer/import-guard Dash-related imports in _main() and raise a clear error like “install with tontester[consensus_explorer] / uv run --extra consensus_explorer ...”.