Cast tzArg to VARCHAR in DATETIME 2-arg column path

Calcite infers CHAR(N) from a literal's length, but the convert_tz yaml
binds tz operands as unbounded `string` (VARCHAR). Without the cast, a
user-supplied tz literal like `'+05:30'` lowers as
`convert_tz(precision_timestamp, char<6>, char<6>)` — same CHAR-vs-string
mismatch already fixed for the synthesised target tz literal, just from
the other side. Cast the source tz to VARCHAR for symmetry; the literal
target was already cast.

Signed-off-by: Vinay Krishna Pudyodu <vinkrish.neo@gmail.com>

Author: vinaykpud

sandbox/plugins/analytics-backend-datafusion/src/main/java/org/opensearch/be/datafusion/DateTimeAdapters.java

@@ -243,12 +243,17 @@ private RexNode adaptTwoArg(RexCall original, RelOptCluster cluster) {
                 strippedValue = value;
             }
             RexNode asTimestamp = rexBuilder.makeCall(original.getType(), LOCAL_TO_TIMESTAMP_OP, List.of(strippedValue));
-            // makeLiteral(String) infers CHAR(N) from the literal's length; the convert_tz yaml
-            // binds the 2nd/3rd operands as `string` (unbounded VARCHAR), so a CHAR(6) "+00:00"
-            // would fail isthmus signature lookup (`convert_tz(precision_timestamp, char<6>, string)`).
+            // The convert_tz yaml binds tz operands as unbounded `string` (VARCHAR). User-supplied
+            // tz literals come in as CHAR(N) (Calcite infers length from the literal), and our
+            // synthesised "+00:00" target would also be CHAR(6) without an explicit type — both
+            // would mis-bind as `convert_tz(precision_timestamp, char<6>, char<6>)`. Cast both to
+            // VARCHAR so isthmus picks the (precision_timestamp, string, string) arm.
             RelDataType varchar = rexBuilder.getTypeFactory().createSqlType(SqlTypeName.VARCHAR);
+            RexNode tzArgVarchar = SqlTypeName.CHAR_TYPES.contains(tzArg.getType().getSqlTypeName())
+                ? rexBuilder.makeCast(varchar, tzArg, true)
+                : tzArg;
             RexNode toTz = rexBuilder.makeLiteral("+00:00", varchar, true);
-            return rexBuilder.makeCall(original.getType(), ConvertTzAdapter.LOCAL_CONVERT_TZ_OP, List.of(asTimestamp, tzArg, toTz));
+            return rexBuilder.makeCall(original.getType(), ConvertTzAdapter.LOCAL_CONVERT_TZ_OP, List.of(asTimestamp, tzArgVarchar, toTz));
         }
 
         /** Plan-time fold of DATETIME(value-literal, tz-literal). Returns a typed TIMESTAMP literal or NULL on invalid input. */