test: add snippet tests for json types system doc

[chelsea-lin]

Fixes internal issue 445529690🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 4 region tags.

• samples/snippets/type_system_test.py:56, tag bigquery_dataframes_type_system_simple_json
• samples/snippets/type_system_test.py:85, tag bigquery_dataframes_type_system_mixed_json
• samples/snippets/type_system_test.py:307, tag bigquery_dataframes_type_system_json_query
• samples/snippets/type_system_test.py:329, tag bigquery_dataframes_type_system_json_query_array

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