feat(pkg-py): add multi-table support

Add the ability to register multiple tables with a QueryChat instance,
enabling cross-table queries and per-table state management.

- **Storage**: Change from single `_data_source` to `_data_sources` dict
- **API**: Add `table_names()`, `table()`, `add_table()`, `remove_table()` methods
- **TableAccessor**: New class for per-table access (`qc.table("name").df()`)
- **Backwards compatible**: Single-table usage works unchanged

- `update_dashboard` and `reset_dashboard` now include table name parameter
- `tool_query` supports querying across multiple tables
- Updated tool prompts to guide LLM on multi-table operations

- Schema section now includes all registered tables
- Added relationship hints for JOINs
- Table descriptions can be provided for LLM context

All frameworks (Shiny, Dash, Streamlit, Gradio) updated to work with
the new dict-based storage while maintaining single-table behavior.

- Added 22 new tests for multi-table functionality
- Updated existing tests for new internal structure

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: cpsievert

.gitignore

@@ -3,6 +3,7 @@ animation.screenflow/
 README_files/
 README.html
 .DS_Store
+test-results/
 python-package/examples/titanic.db
 .quarto
 *.db