feat(budgetTracker): add onOverspend callback when spending exceeds budget ceiling

[rupesh-kumar-sah]

Summary of Changes

Resolves #704

This PR adds an onOverspend(overspentBy: number, budgetId: string) callback option to budgetTracker:

1. Configuration Acceptance:
   • onOverspend callback is accepted via configureBudgetTracker(config), setBudgetLimit(address, limit, options), BudgetTracker class configuration, or recordPayment(address, amount, options).
2. Exact Trigger Semantics:
   • The callback fires exactly once per payment that crosses the budget ceiling (prevSpent <= limit && newSpent > limit). Subsequent payments made while the budget is already exceeded do not trigger repeated callbacks.
3. Difference Calculation:
   • overspentBy is calculated as the exact positive difference between the new total spent and the budget ceiling in USDC.
4. Backward Compatibility & Verification:
   • All existing tests pass without regressions.
   • Added unit test suite covering ceiling crossing, subsequent payments exclusion, positive difference calculation, and configuration options.

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