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fix(compiler): add left shift overflow check in checked context (#1486)

* fix(compiler): add left shift overflow check in checked context

- Add CheckShiftOverflow method to validate shift amount
- Prevent unexpectedly large values from left shift operations
- Runtime check throws if shift amount exceeds type bit width

Fixes HIGH-001 from compiler security audit

* fix: call CheckShiftOverflow for left shift operator in checked context

- Add call to CheckShiftOverflow when processing '<<' operator
- Add checked context guard in CheckShiftOverflow method

Addresses review comment: https://github.com/neo-project/neo-devpack-dotnet/pull/1486#discussion_r2732283515

* fix: treat BigInteger as Int256 (256-bit) for shift overflow check

In NEO, BigInteger is Int256, not unlimited precision.
Updated maxShift from -1 (no limit) to 256 bits.

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