Optimize parse_in_bracket to avoid unnecessary String allocations

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**

I've noticed that `parse_in_bracket` in `parquet-variant/src/utils.rs` always allocates a `String` when parsing bracket expressions like `[field]` or `[0]`, even when no escaping is needed:

```rust
fn parse_in_bracket(s: &str, i: usize) -> Result<(VariantPathElement<'_>, usize), ArrowError> {
let start = i + 1;
let mut unescaped = String::new(); // <-- Always allocates!
// ...
}
```

**Describe the solution you'd like**

I'm thinking of optimizing this by:

1. Check if the content contains `\` before allocating
2. Since `VariantPathElement` already supports `Cow<'a, str>`, we could borrow the slice directly when no escaping is present
3. Only allocate when we actually encounter escape sequences

Start in parquet-variant/src/utils.rs at parse_in_bracket and inspect how VariantPathElement uses Cow. Compare bracket expressions with and without escape sequences, then verify that unescaped content is borrowed, escaped content still parses correctly, and the existing parquet-variant tests remain passing.

Written by the indexing model from the issue text.