Fix string escape sequences in lexer

[waterdrag0n]

Problem

The lexer doesn't handle escape sequences in string literals, causing parser failures when using features like additional_table_filters with quoted values.

Example that fails:

SELECT * FROM logs
SETTINGS additional_table_filters={'logs': 'level = \'ERROR\''};

Error: expected the last token kind is: }, but got <ident>

Solution

Updated consumeString() in the lexer to handle both ClickHouse-standard escape methods:

• Backslash escapes: '
• Double single quotes: ''

[coveralls]

Pull Request Test Coverage Report for Build 19030368812

Details

• 16 of 16 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 50.619%

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Totals
Change from base Build 18928213117:	0.06%
Covered Lines:	7641
Relevant Lines:	15095

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💛 - Coveralls

[git-hulk]

LGTM.

@waterdrag0n Thanks for your good catch and fix.