test(examples): add pytest tests for mathkit Calculator (ABCA-733)

[isadeks]

Summary

ABCA-733 — [mathkit-3 depends-on-2] Adds examples/mathkit/test_calculator.py with pytest tests that import Calculator from the sibling calculator.py module (ABCA-732).

This is the third step of the mathkit dependency chain:

• mathkit-1 (ABCA-731): operations.py
• mathkit-2 (ABCA-732): calculator.py — this PR is based on that branch
• mathkit-3 (ABCA-733): test_calculator.py — this PR

Tests added

15 test cases covering the Calculator class:

• Default and configurable initial value
• Each arithmetic method (add, subtract, multiply, divide)
• Divide-by-zero raises ValueError
• reset() to default and to a specific value
• Method chaining returns self
• The chained example from the docstring (Calculator(2).add(3).multiply(4).value == 20.0)
• A longer chain and a parametrized add case

Verification

Run from within examples/mathkit:

cd examples/mathkit && pytest

Result: 15 passed.

Notes

• Base branch is the mathkit-2 branch (not the default branch) because this PR depends on calculator.py, which isn't merged yet. Retarget once mathkit-2 lands.
• The pre-push SAST hook flags two pre-existing findings in unrelated files (cdk/src/handlers/shared/orchestration-comment-trigger.ts, scripts/linear_epic.py) that this change does not touch.

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