fix: unwrap BigQuery REPEATED array elements during decoding

[I-chat]

Summary

• BigQuery wraps each element of a REPEATED array in its own {"v": ...} cell object, but BQCellValue.init(from:) decoded elements as bare values — every element spuriously matched .record(f: nil) and rendered as null, so any STRING REPEATED (or repeated STRUCT) column showed as [null, null, ...].
• Array elements now decode as [BQQueryResponse.BQCell] and unwrap .v, so each element resolves to its real .string/.record/.null case.

Test plan

• Added BigQueryTypeMapperJSONDecodingTests covering REPEATED STRING and REPEATED RECORD, decoding BigQuery's actual wrapped-array JSON shape end-to-end
• Confirmed the new tests fail without the fix (reverted it locally, reran) and pass with it
• Full TableProTests suite passes
• Verified live against a real BigQuery table with a STRING REPEATED column