Fix conv_general output offset overflow in Metal writeback

[Aristide021]

Fixes incorrect output writes in Metal conv_general when output offsets exceed signed 32-bit range.

Root cause

In Metal steel conv writeback paths, output pointer offset arithmetic used 32-bit int intermediates. For large outputs, this can overflow and corrupt writes.

What this changes

• Switches output offset arithmetic to 64-bit (size_t) in:
  • mlx/backend/metal/kernels/steel/conv/kernels/steel_conv.h
  • mlx/backend/metal/kernels/steel/conv/kernels/steel_conv_general.h
• Keeps the change scoped to kernel writeback offset calculation (no API/semantic changes)

Tests

• Adds C++ regression test in tests/ops_tests.cpp:
  • test conv2d output offset uses 64-bit arithmetic
  • Verifies offset math past int32 max
  • Verifies mismatch vs prior int32 wraparound behavior

Validation run

• cmake --build build --target mlx-metallib
• cmake --build build --target tests
• build/tests/tests --test-case "test conv2d output offset uses 64-bit arithmetic"
• Reproducer threshold checks (pointwise conv_general vs matmul) now pass for cases above 2^31

Closes #3248.

[zcbenz]

The added test passes without the change.

[Aristide021]

The added test passes without the change.

I tried to build the e2e test but found a blocker unrelated to this fix. I've filed a separate issue: #3327.

I'm closing this PR for now since I can't meaningfully verify the fix with a reliable regression test. Once #3327 is resolved, this can be revisited with proper coverage.