Revert deparse.c duckdb_quote_identifier changes

deparse.c output is only used as an intermediate SQL string passed to
ParseQuery() to reconstruct a Query AST; it is never sent to DuckDB
directly.  The AST is then re-deparsed by PreparePGDuckSQLTemplate /
pg_get_querydef, whose output is post-processed by
RequoteDuckDBReservedInSQL.  The duckdb_quote_identifier calls in
deparse.c are therefore dead code for DuckDB quoting purposes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: David Christensen <david.christensen@snowflake.com>

Author: sfc-gh-dachristensen

pg_lake_table/src/fdw/deparse.c

@@ -41,16 +41,6 @@
  * with collations that match the remote table's columns, which we can
  * consider to be user error.
  *
- * Identifier quoting in this file uses duckdb_quote_identifier() rather than
- * quote_identifier() because all SQL generated here is sent to pgduck_server
- * (DuckDB), not PostgreSQL.  DuckDB reserves additional keywords (LAMBDA,
- * PIVOT, QUALIFY, etc.) that PostgreSQL does not; duckdb_quote_identifier()
- * handles those.
- *
- * TODO: this file was ported from postgres_fdw and has not been fully audited
- * for other DuckDB SQL compatibility issues (e.g. operator syntax, type casts).
- * See https://github.com/Snowflake-Labs/pg_lake/issues/277.
- *
  * Portions Copyright (c) 2012-2023, PostgreSQL Global Development Group
  *
  *-------------------------------------------------------------------------
@@ -91,7 +81,6 @@
 #include "pg_lake/extensions/postgis.h"
 #include "pg_lake/fdw/pg_lake_table.h"
 #include "pg_lake/fdw/shippable.h"
-#include "pg_lake/pgduck/keywords.h"
 #include "pg_lake/pgduck/type.h"
 #include "pg_lake/pgduck/rewrite_query.h"
 #include "pg_lake/planner/restriction_collector.h"
@@ -2231,7 +2220,7 @@ deparseAnalyzeSql(StringInfo buf, Relation rel,
 			}
 		}
 
-		appendStringInfoString(buf, duckdb_quote_identifier(colname));
+		appendStringInfoString(buf, quote_identifier(colname));
 
 		*retrieved_attrs = lappend_int(*retrieved_attrs, i + 1);
 	}
@@ -2469,7 +2458,7 @@ deparseColumnRef(StringInfo buf, int varno, int varattno, RangeTblEntry *rte,
 		if (qualify_col)
 			ADD_REL_QUALIFIER(buf, GetUniqueRelationIdentifier(rte));
 
-		appendStringInfoString(buf, duckdb_quote_identifier(colname));
+		appendStringInfoString(buf, quote_identifier(colname));
 	}
 }
 
@@ -2512,7 +2501,7 @@ deparseRelation(StringInfo buf, Relation rel)
 		relname = RelationGetRelationName(rel);
 
 	appendStringInfo(buf, "%s.%s",
-					 duckdb_quote_identifier(nspname), duckdb_quote_identifier(relname));
+					 quote_identifier(nspname), quote_identifier(relname));
 }
 
 /*
@@ -3106,7 +3095,7 @@ deparseOperatorName(StringInfo buf, Form_pg_operator opform)
 		opnspname = get_namespace_name(opform->oprnamespace);
 		/* Print fully qualified operator name. */
 		appendStringInfo(buf, "OPERATOR(%s.%s)",
-						 duckdb_quote_identifier(opnspname), opname);
+						 quote_identifier(opnspname), opname);
 	}
 	else
 	{
@@ -3532,7 +3521,7 @@ deparseFieldSelect(FieldSelect *fieldSelect, deparse_expr_cxt *context)
 					attr_index,
 					false);
 
-	appendStringInfoString(context->buf, duckdb_quote_identifier(field_name));
+	appendStringInfoString(context->buf, quote_identifier(field_name));
 }
 
 /*
@@ -3767,12 +3756,12 @@ appendFunctionName(Oid funcid, deparse_expr_cxt *context)
 		const char *schemaname;
 
 		schemaname = get_namespace_name(procform->pronamespace);
-		appendStringInfo(buf, "%s.", duckdb_quote_identifier(schemaname));
+		appendStringInfo(buf, "%s.", quote_identifier(schemaname));
 	}
 
 	/* Always print the function name */
 	proname = NameStr(procform->proname);
-	appendStringInfoString(buf, duckdb_quote_identifier(proname));
+	appendStringInfoString(buf, quote_identifier(proname));
 
 	ReleaseSysCache(proctup);
 }