#23816 fix(chart-tools): ensure configurable dict mutation and coerce data points (#23287)

Summary

Closes #23287

Resolves two critical issues in create_chart_tool (backend/utils/retrieval/tools/chart_tools.py):

1. Configurable binding & mutation:
   • Previously, configurable = config.get('configurable', {}) retrieved a detached temporary dictionary when the 'configurable' key was missing, silently mutating the temporary dictionary while leaving config unchanged (inline chart dropped from response).
   • If config contained {'configurable': None}, config.get('configurable', {}) returned None, raising TypeError: 'NoneType' object does not support item assignment.
   • Now safely ensures config['configurable'] is a dict before setting chart_data.
2. Defensive data point coercion:
   • Defensively coerces data point labels to strings and values to floats (fallback to 0.0 on invalid/null/bool), preventing wire schema and downstream serialization errors.
3. Hermetic unit tests:
   • Added backend/tests/unit/test_chart_tools.py with 5 tests covering configurable persistence, None handling, input coercion, validations, and execution without context.

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