[wgtmac]

Rationale for this change

Fix UB in StructConverter::AppendValue(), which reused a simdjson ondemand value after get(array) failed.

See #55497.

What changes are included in this PR?

• Determine the JSON type before calling get() in BooleanConverter, TimestampConverter, and StructConverter.
• Refactor GetJsonAs() and IsJsonNull() to avoid accessing a value after a failed typed access.

Are these changes tested?

Added a case matching the nested struct/object notation from the issue.

Are there any user-facing changes?

No.

Was AI used for this PR?

PR code and description written by:

• Human
• AI

Reviewed before submission by:

• Human
• AI
• Not reviewed

• GitHub Issue: [C++][JSON] Avoid reusing simdjson values after failed get() #55497

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[wgtmac]

Our internal integration crashed due to this UB. I've proposed a fix, please take a look when you have time. Thanks! @kou @pitrou @Reranko05