Fixes #6243 - Strange behavior for duplicate imports with imports_granularity="Module"

Problem

When using imports_granularity="Module", duplicate imports were being merged but not deduplicated, requiring multiple formatting passes to reach a stable state. This was inconsistent with other granularity settings which properly remove duplicates.

Solution

Added .dedup() calls after sorting import lists in three locations:

• merge_rest() - when creating merged import lists
• UseTree::normalize() - when normalizing nested imports
• merge_use_trees_inner() - when merging use trees

This ensures duplicate imports are removed in a single formatting pass, matching the behavior of Crate and Item granularity settings.

Test Changes

Updated tests/target/multiple.rs to expect deduplicated imports. I believe the previous test was documenting the buggy behavior rather than the expected behavior.

Example

Before (unstable, requires 2 passes):

use risingwave_batch::executor::{BoxedExecutor, JoinType};
use risingwave_batch::executor::{BoxedExecutor, JoinType};
// First pass: {BoxedExecutor, BoxedExecutor, JoinType, JoinType}
// Second pass: {BoxedExecutor, JoinType}

After (stable, single pass):

use risingwave_batch::executor::{BoxedExecutor, JoinType};