test: add overflow tests for MutableBuffer

[SoimanVasile]

Description

While analyzing the memory allocation logic in MutableBuffer, I identified that the with_capacity and reserve methods correctly use .expect() guards to prevent integer overflows.

However, I couldnt find test cases for this .expect() guard in the current test suite. This PR adds 2 #[should_panic] tests in mutable.rs to verify that the API correctly panics
Changes:

• Added test_mutable_new_capacity_overflow to cover MutableBuffer::new
• Added test_mutable_reserve_overflow to cover MutableBuffer::reserve

Rationale

Adding these tests ensures that the safety guards in arrow-buffer remain intact and provides explicit coverage for edge cases involving near-usize::MAX allocations.

Tests

• test_mutable_new_capacity_overflow
• test_mutable_reserve_overflow