[Rust] Reset apparent_size in Verifier::reset()

[xhon-pelushi]

Fixes #9189.

Verifier::reset() assigns num_tables twice and never clears apparent_size:

pub fn reset(&mut self) {
    self.depth = 0;
    self.num_tables = 0;
    self.num_tables = 0;      // apparent_size was meant here
}

apparent_size is initialised in new() and incremented in range_in_buffer(), but cleared nowhere. A Verifier reused through reset() therefore keeps accumulating across buffers, and once the running total passes max_apparent_size it rejects every subsequent buffer with ApparentSizeTooLarge, however small or valid that buffer is. In other words reset() leaves the verifier in a state new() can never produce, which is the opposite of what it is for.

Fix

Assign apparent_size in place of the duplicated num_tables line, plus a test that spends most of the budget, resets, and spends it again. The test is inline as #[cfg(test)] mod tests, matching builder.rs.

Against master it fails as:

assertion `left == right` failed: reset() must clear apparent_size, not just depth and num_tables
  left: Err(ApparentSizeTooLarge)
 right: Ok(())

Testing

• cargo test -p flatbuffers — 7 passed after the change (6 before, plus the new one).
• tests/rust_usage_test — full suite green: 276 + 21 + 13 and the smaller suites, 0 failures.

One thing to be aware of, unrelated to this change: builder::tests::with_internal_capacity_preallocates_vecs already fails on master with "expected 0 allocations after builder construction, got 2". I confirmed that by stashing this patch and re-running, so please don't attribute it to this PR.