[Enhancement] Rewrite element_at(split_by_string(s,sep),n) to split_part in filter conjuncts

### Search before asking

- [X] I had searched in the [issues](https://github.com/apache/doris/issues?q=is%3Aissue) and found no similar issues.

### Description

`element_at(split_by_string(s, sep), n)` materializes the entire split array (a PODArray allocation plus a memcpy per part, per row) and then selects a single entry. When such an expression is the root of a top-level filter conjunct, it can be rewritten to `split_part(s, sep, n)`, which short-circuits as soon as it locates the n-th delimiter — eliminating the intermediate array and the trailing-parts work.

The rewrite must be semantics-preserving. The two forms differ only when the string yields no n-th part: `element_at` returns NULL while `split_part` can return `''`. This can be neutralized by restricting the rewrite to the root of a top-level filter conjunct (where NULL and false both drop the row) and guarding it (non-empty string-literal comparison RHS, non-empty literal separator, non-negative literal index), and by not recursing into NOT / OR / CASE where NULL and `''` are no longer interchangeable.

### Use case

Queries filtering on a specific token of a delimited string column, e.g. `WHERE split_by_string(col, ',')[1] = 'x'`, run a per-row array build today; the rewrite makes them scan-friendlier.

### Related issues

_No response_

### Are you willing to submit PR?

- [X] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

The issue names no files or tests. Start by locating Doris's filter-conjunct expression-rewrite or optimizer entry point and the handling for split_by_string, element_at, and split_part; done means the guarded top-level-conjunct rewrite preserves the stated NULL behavior and excludes NOT, OR, and CASE contexts.

Written by the indexing model from the issue text.