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

[cpsievert]

Summary

This PR adds multi-table support to QueryChat Python package, allowing users to chat with multiple related tables in a single session.

Key Features

• Multi-table API: add_table(), remove_table(), table_names(), table("name") methods
• TableAccessor: Per-table access via qc.table("name").df(), .sql(), .title(), .ui()
• Per-table state: Independent SQL/title per table
• Backwards compatible: Single-table usage remains unchanged
• Integrates with deferred initialization: Works with the new data_source setter pattern

API Examples

# Add multiple tables
qc = QueryChat(orders_df, "orders", greeting="Hello!")
qc.add_table(customers_df, "customers", relationships={"id": "orders.customer_id"})

# Access per-table state
qc.table("orders").df()
qc.table("customers").sql()

# List tables
qc.table_names()  # ["orders", "customers"]

Core Changes

File	Changes
_querychat_base.py	_data_sources dict storage, add_table(), remove_table(), table_names(), table()
_table_accessor.py	New file with TableAccessor class
_system_prompt.py	Multi-source support with relationships
tools.py	Table parameter on update/reset tools, dict-based data sources
prompts/*.md	Multi-table instructions and table parameter docs

Design Document

See docs/plans/2025-01-14-multi-table-design.md for the full design specification.

Test plan

• All 292 tests pass (including new deferred initialization tests from main)
• New tests in test_multi_table.py cover storage, add/remove, accessor, ambiguity errors
• Ruff linting passes
• Manual testing with multi-table app

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