[fix](varaint) fix doc mode regresssion case (#61716)

### What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

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- Test <!-- At least one of them must be included. -->
    - [x] Regression test
    - [ ] Unit Test
    - [ ] Manual test (add detailed scripts or steps below)
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Author: csun5285

regression-test/suites/variant_p1/doc_snapshot/load.groovy

@@ -66,7 +66,7 @@ suite("predefine_type_multi_index_doc_value", "p1"){
                 MATCH_NAME 'actor.login' : string,
                 MATCH_NAME 'type' : string,
                 MATCH_NAME 'payload.action' : string,
-                MATCH_NAME 'created_at' : datetime,
+                MATCH_NAME 'created_at' : string,
                 MATCH_NAME 'payload.issue.number' : int,
                 MATCH_NAME 'payload.comment.body' : string,
                 MATCH_NAME 'type.name' : string