fix(rust): Zero vtable memory in write_vtable to prevent uninitialized data (#8898)

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").

This fix explicitly zeros the 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.

Fixes #8894

rust/flatbuffers/src/builder.rs

@@ -611,6 +611,8 @@ impl<'fbb, A: Allocator> FlatBufferBuilder<'fbb, A> {
         // Write the VTable (we may delete it afterwards, if it is a duplicate):
         let vt_start_pos = self.head;
         let vt_end_pos = self.head + vtable_byte_len;
+        // Zero out the vtable space - make_space only reserves but doesn't initialize
+        self.allocator[vt_start_pos.range_to(vt_end_pos)].fill(0);
         {
             // write the vtable header:
             let vtfw =