fix: resolve scalafix and q9 plan-stability failures after main merge

Two issues surfaced once CI ran (the feature commits used [skip ci]):

- datetime.scala: drop redundant `s` interpolators on two string literals in
  the date_format info hint (scalafix CHECK).
- tpcds q9 golden: the info-message change makes ExtendedExplainInfo omit an
  empty `[COMET: ]` for a bare Spark fallback Project, so the root renders as
  `Project`. Update the approved plan to match.

Author: andygrove

spark/src/main/scala/org/apache/comet/serde/datetime.scala

@@ -688,9 +688,9 @@ object CometDateFormat extends CometExpressionSerde[DateFormatClass] {
         // Native `to_char` could run this if allowed; tell the user about the faster path.
         withInfo(
           expr,
-          s"a faster native implementation is available; set " +
+          "a faster native implementation is available; set " +
             s"${CometConf.getExprAllowIncompatConfigKey(getExprConfigName(expr))}=true " +
-            s"to enable it (results may differ from Spark for non-UTC time zones)")
+            "to enable it (results may differ from Spark for non-UTC time zones)")
       }
       // Hand the full `DateFormatClass` (with `timeZoneId` already stamped by `ResolveTimeZone`)
       // to the codegen dispatcher. It closure-serializes the bound tree, so non-UTC timezones

spark/src/test/resources/tpcds-plan-stability/approved-plans-v1_4/q9/extended.txt

@@ -1,4 +1,4 @@
- Project [COMET: ]
+Project
 :  :- Subquery
 :  :  +- CometNativeColumnarToRow
 :  :     +- CometProject