fix(rust): Zero vtable memory in write_vtable to prevent uninitialize…

[brianmacy]

Summary

Fixes #8894

The write_vtable() function's comment claimed to "fill the WIP vtable with zeros" but make_space() only reserves memory without initializing it. When using custom allocators with non-zeroed buffers, unset vtable field entries would contain garbage instead of zero (which indicates "use default value").

Changes:

• Added explicit zeroing of vtable memory after reserving space, matching the C++ implementation's buf_.fill_big() behavior
• Added regression test using a garbage-filled allocator (0xAA) that verifies vtable entries for unset fields are properly zeroed

Test plan

• New unit test test_vtable_zeroed_with_garbage_allocator added
• Test fails without fix (shows 0xAAAA garbage in vtable)
• Test passes with fix
• All existing Rust tests pass

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[jtdavis777]

I don't think any of our other tests are written in source -- is there a way to test this from the tests directory? Or do you need intimate access to create the allocator? I am not a rust person :)

[brianmacy]

I barely know Rust either. This is just the way I've seen it done. I'm sure I can follow the pattern of the others if needed.

[brianmacy]

I think that resolves it.

[brianmacy]

I don't think any of our other tests are written in source -- is there a way to test this from the tests directory? Or do you need intimate access to create the allocator? I am not a rust person :)

Now that tat is fixed, let me know if there is anything else I need to do.