Implement FT.EXPLAIN and FT.EXPLAINCLI for query debugging

[vyavdoshenko]

Implement query explanation commands for debugging search queries. FT.EXPLAIN {index} {query} [DIALECT {dialect}]

• Parse query using existing SearchParser
• Return AST as formatted string
• Shows how query is interpreted (INTERSECT, UNION, tags, etc.)

FT.EXPLAINCLI {index} {query} [DIALECT {dialect}]

• Same as FT.EXPLAIN but with CLI-friendly formatting
• Add indentation and visual tree structure