Use checked rounding for BFC arena allocations

[Akshay Sonawane (apsonawane)]

This pull request improves the safety and robustness of memory allocation in the BFCArena allocator by adding overflow protection and corresponding tests. The most important changes are:

Overflow Protection and Safety:

• Updated the BFCArena::RoundedBytes function in bfc_arena.cc to use SafeInt<size_t> for arithmetic operations, preventing integer overflows during memory rounding calculations.
• Included the safeint.h header to enable safe integer operations in bfc_arena.cc.

Testing:

• Added a new test, RoundedBytesOverflowThrows, in bfc_arena_test.cc to verify that an overflow during allocation throws an OnnxRuntimeException.
• Included the <limits> header in bfc_arena_test.cc to support boundary value tests.

[Copilot]

Pull request overview

This PR hardens BFCArena’s allocation-size rounding logic against integer overflow by switching the rounding arithmetic to SafeInt<size_t>, and adds a unit test to ensure overflow cases throw an OnnxRuntimeException instead of silently wrapping.

Changes:

• Updated BFCArena::RoundedBytes to use SafeInt<size_t> for checked rounding arithmetic (preventing overflow on bytes + kMinAllocationSize - 1).
• Added a regression test validating that an overflow-sized allocation request throws OnnxRuntimeException.
• Added required headers to support the new checked arithmetic and boundary-value test.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
onnxruntime/core/framework/bfc_arena.cc	Uses SafeInt<size_t> in BFCArena::RoundedBytes to make rounding overflow-safe.
onnxruntime/test/framework/bfc_arena_test.cc	Adds RoundedBytesOverflowThrows to assert overflow requests throw OnnxRuntimeException.

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