fix: handle DECIMAL(38,9) type conversion in BigQuery processor

[devin-ai-integration]

Fix BigQuery DECIMAL(38,9) type conversion

This PR fixes the issue where BigQuery cache operations fail with "Invalid value for type: DECIMAL(38, 9)" error by properly mapping SQLAlchemy DECIMAL types to BigQuery's NUMERIC type with appropriate precision and scale parameters.

Changes:

• Added explicit handling of DECIMAL types in BigQueryTypeConverter
• Maps SQLAlchemy DECIMAL to BigQuery NUMERIC(38,9)

Testing:

• Manual testing needed to verify the fix works with actual BigQuery cache operations

Fixes #573

Link to Devin run: https://app.devin.ai/sessions/a5b1089b2e6749039d54de93a56e7c39

[devin-ai-integration]

Original prompt:

@Devin - Can you investigate this one? Per the reporters most recent comment, we should be able to use the pre- and post-manifesting version diff for clues. <https://github.com/airbytehq/pyairbyte/issues/573>

[devin-ai-integration]

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