Add descriptive ValueError when no operators are added to ComponentGenerator

[karlhillx]

Fixes #90.

Problem

When ComponentGenerator is used without calling add_operator(), random_operator() raises a cryptic IndexError: list index out of range from deep in ProbabilityMassFunction.draw_sample(). The error gives users no indication of what went wrong or how to fix it.

Fix

Add a guard in random_operator() that checks if _operator_pmf.items is empty and raises a descriptive ValueError instead:

ValueError: No operators have been added to this ComponentGenerator. Call add_operator() before generating graphs.

Since random_operator() is the single choke point for all operator-drawing paths (direct calls, random_operator_command(), and random_command() → PMF → random_operator_command()), the guard catches all entry paths with one check.

Tests

Added TestNoOperatorsError class with three tests:

• test_random_operator_raises_without_operators — direct call to random_operator()
• test_random_operator_command_raises_without_operators — via random_operator_command()
• test_generator_call_raises_without_operators — end-to-end replication via AGraphGenerator.__call__

All 31 tests in test_component_generator.py pass. The 2 pre-existing failures in test_expression.py (sklearn __sklearn_tags__ compatibility) are unrelated to this change.

[Copilot]

Pull request overview

This PR improves the usability of AGraph expression generation by replacing a cryptic IndexError (triggered when no operators are configured) with a clear, actionable ValueError from ComponentGenerator.random_operator(), and adds regression tests for the affected call paths.

Changes:

• Add an explicit guard in ComponentGenerator.random_operator() that raises a descriptive ValueError when no operators have been added.
• Extend the random_operator() docstring with a Raises section documenting the new behavior.
• Add unit tests covering direct operator selection, operator-command generation, and end-to-end generator invocation when no operators are configured.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
bingo/expressions/agraph/component_generator.py	Adds the empty-operator guard and documents the new exception behavior.
tests/unit/expressions/agraph/test_component_generator.py	Adds regression tests ensuring all entry paths raise the descriptive ValueError when no operators are configured.

[gbomarito]

Thanks for the work!