Handle unicode whitespace characters in lexer

Add isClickHouseWhitespace() to recognize characters that ClickHouse
treats as whitespace but Go's unicode.IsSpace() does not: U+FEFF BOM,
U+180E Mongolian Vowel Separator, U+200B Zero Width Space, U+200C Zero
Width Non-Joiner, U+200D Zero Width Joiner, and U+2060 Word Joiner.

This fixes the 01280_unicode_whitespaces_lexer test case stmt3.

Author: claude

lexer/lexer.go

@@ -73,12 +73,33 @@ func (l *Lexer) peekChar() rune {
 }
 
 func (l *Lexer) skipWhitespace() {
-	// Skip whitespace and BOM (byte order mark U+FEFF)
-	for unicode.IsSpace(l.ch) || l.ch == '\uFEFF' {
+	// Skip whitespace, BOM, and other Unicode characters that ClickHouse treats as whitespace.
+	// See: https://github.com/ClickHouse/ClickHouse/blob/master/src/Parsers/Lexer.cpp
+	for unicode.IsSpace(l.ch) || isClickHouseWhitespace(l.ch) {
 		l.readChar()
 	}
 }
 
+// isClickHouseWhitespace returns true for characters ClickHouse treats as whitespace
+// but Go's unicode.IsSpace does not recognize.
+func isClickHouseWhitespace(ch rune) bool {
+	switch ch {
+	case '\uFEFF': // BOM (Byte Order Mark)
+		return true
+	case '\u180E': // MONGOLIAN VOWEL SEPARATOR
+		return true
+	case '\u200B': // ZERO WIDTH SPACE
+		return true
+	case '\u200C': // ZERO WIDTH NON-JOINER
+		return true
+	case '\u200D': // ZERO WIDTH JOINER
+		return true
+	case '\u2060': // WORD JOINER
+		return true
+	}
+	return false
+}
+
 // NextToken returns the next token from the input.
 func (l *Lexer) NextToken() Item {
 	l.skipWhitespace()

parser/testdata/01280_unicode_whitespaces_lexer/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt3": true
-  }
-}
+{}