fix: set escape_char when simplifying starts_with to LIKE

[jackye1995]

Summary

The starts_with simplification escapes special LIKE characters (%, _, \) with backslash but did not set escape_char in the resulting LIKE expression. This caused the escaped characters to be treated as literals instead of escape sequences.

Problem

When starts_with(col, 'test_ns') is simplified to col LIKE 'test\_ns%', the escape_char was set to None. This meant the \_ was not recognized as an escaped underscore, and the underscore was incorrectly interpreted as a LIKE wildcard matching any single character.

Example

-- Input
starts_with(namespace, 'test_ns$')

-- Before fix (incorrect)
namespace LIKE 'test\_ns$%'  -- escape_char: None
-- The `_` is treated as wildcard, matching any char

-- After fix (correct)  
namespace LIKE 'test\_ns$%'  -- escape_char: Some('\\')
-- The `\_` is correctly interpreted as literal underscore

Fix

Set escape_char: Some('\\') when creating the LIKE expression, since the simplification already uses backslash to escape special characters.

Testing

• Existing starts_with tests pass
• Verified fix resolves query correctness issues in downstream projects (Lance)

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