LAV-2499: Support ALTER DYNAMIC TABLE column comments (#2892)

* LAV-2499: support dynamic table column comments

Co-authored-by: Sabir Akhadov <sabir.akhadov@localstack.cloud>

* LAV-2499: reject identifier column comments

Restrict dynamic-table column comments to Snowflake single-quoted and dollar-quoted string literals, with real-Snowflake snapshots for bare and double-quoted identifier errors.\n\n## Deviations\n\n- Reviewer feedback requested that ALTER DYNAMIC TABLE column-comment actions reject ordinary tables. A real-Snowflake snapshot proves this spelling succeeds and updates the ordinary table column comment, so the existing behavior is retained as required by the repository's snapshot precedence rule.

Co-authored-by: Sabir Akhadov <sabir.akhadov@localstack.cloud>

* LAV-2499: snapshot column comment errors

Route dynamic-table column-comment failures through the wire-level snapshot fixture and align the missing-object IF EXISTS source position with Snowflake.

Co-authored-by: Sabir Akhadov <sabir.akhadov@localstack.cloud>

---------

Co-authored-by: spiral <spiral@localhost>
Co-authored-by: Sabir Akhadov <sabir.akhadov@localstack.cloud>
Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

Author: 4 people

src/ast/ddl.rs

@@ -1444,7 +1444,6 @@ pub enum AlterColumnOperation {
         /// The comment text.
         comment: String,
     },
-
     /// `UNSET COMMENT`
     ///
     /// Snowflake: clear the column comment.

src/dialect/snowflake.rs

@@ -1607,20 +1607,31 @@ fn parse_alter_dynamic_table_column_comments(
 ) -> Result<Vec<AlterTableOperation>, ParserError> {
     let _ = parser.next_token(); // ALTER | MODIFY
     let parenthesized = parser.consume_token(&Token::LParen);
-    let operations = parser.parse_comma_separated(|parser| {
+    let mut operations = Vec::new();
+    loop {
         let _ = parser.parse_keyword(Keyword::COLUMN);
         let column_name = parser.parse_identifier()?;
         let op = if parser.parse_keyword(Keyword::COMMENT) {
-            AlterColumnOperation::Comment {
-                comment: parser.parse_literal_string()?,
-            }
+            let token = parser.next_token();
+            let comment = match token.token {
+                Token::SingleQuotedString(value) => value,
+                Token::DollarQuotedString(value) => value.value,
+                _ => return parser.expected("string literal", token),
+            };
+            AlterColumnOperation::Comment { comment }
         } else if parser.parse_keywords(&[Keyword::UNSET, Keyword::COMMENT]) {
             AlterColumnOperation::UnsetComment
         } else {
-            return parser.expected_ref("COMMENT or UNSET COMMENT", parser.peek_token_ref());
+            return parser.expected_ref(
+                "COMMENT or UNSET COMMENT after column name",
+                parser.peek_token_ref(),
+            );
         };
-        Ok(AlterTableOperation::AlterColumn { column_name, op })
-    })?;
+        operations.push(AlterTableOperation::AlterColumn { column_name, op });
+        if !parser.consume_token(&Token::Comma) {
+            break;
+        }
+    }
     if parenthesized {
         parser.expect_token(&Token::RParen)?;
     }