Fix NUMERIC columns being converted to integers (#624)

[byapparov]

Fixes #624

Some BigQuery configurations incorrectly return NUMERIC/BIGNUMERIC columns with INTEGER type in the schema, causing them to be parsed as integer64 and then converted to regular integers when bigint="integer".

This fix ensures NUMERIC and BIGNUMERIC columns are always preserved as doubles regardless of schema inconsistencies, while still allowing true INTEGER columns to be converted according to the bigint parameter.

• Add explicit handling for NUMERIC/BIGNUMERIC columns in parse_postprocess
• Ensure bigint conversion only applies to true INTEGER columns
• Add comprehensive test case to prevent regression

🤖 Generated with Claude Code

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This is created as an experiment, so please ignore if it is not fixing the problem or addresses the wrong issue.