bugfix(formatter): Sort merged items via compare_names. - #10123

Summary

Fixes incorrect leaf ordering when merging use items with allow_duplicate_uses disabled. Previously, leaves were converted to strings before deduplication and sorting, which meant the sort operated on formatted strings (e.g., "x as y") rather than on the structured name/alias fields. This caused self to not be placed first in merged braces groups. Now, sorting and deduplication happen directly on Leaf structs, sorting by name (using compare_names) and then by alias, before formatting. A Display impl and PartialEq/Eq derives were added to Leaf to support this.

A new test case (use_merge_no_sort.cairo) covers the scenario where module-level sorting is disabled but merged braces must still honor the self-first convention.

Type of change

Please check one:

• Bug fix (fixes incorrect behavior)
• New feature
• Performance improvement
• Documentation change with concrete technical impact
• Style, wording, formatting, or typo-only change

Why is this change needed?

When merging use paths with duplicate-use checking enabled, leaves like self, b, and z under the same prefix were not being ordered correctly. The old code sorted string representations of leaves (e.g., "self", "b", "z") after formatting them, but compare_names (which enforces self-first ordering) was never applied to leaves — only to the string-level sort. This meant the output could be {b, self, z} instead of the correct {self, b, z}.

What was the behavior or documentation before?

Given:

use a::z;
use a::self;
use a::b;

The formatter could produce use a::{b, self, z}; instead of use a::{self, b, z}; when merging was enabled but module-level sorting was disabled.

What is the behavior or documentation after?

The formatter now correctly produces:

use a::{self, b, z};

Leaf nodes are sorted using compare_names on their name field (with alias as a tiebreaker) before being formatted and merged into braces groups.

Related issue or discussion (if any)

None.

Additional context

The Leaf struct now derives PartialEq and Eq (required for dedup) and implements Display to avoid duplicating the name as alias formatting logic.