fix: format BigQuery numeric output as decimal strings

[shobsi]

This change updates both bigquery-sql and bigquery-execute-sql tools to format NUMERIC and BIGNUMERIC values as decimal strings (e.g., "9.5") instead of rational fractions (e.g., "19/2"). This ensures the tools' output matches the BigQuery REST API JSON format.

Key changes:

• Added NormalizeValue function in internal/tools/bigquery/bigquerycommon to handle *big.Rat conversion with 38-digit precision and trailing zero trimming.
• Updated bigquery-sql and bigquery-execute-sql to use NormalizeValue.
• Added comprehensive tests in internal/tools/bigquery/bigquerycommon/conversion_test.go.

Description

Should include a concise description of the changes (bug or feature), it's impact, along with a summary of the solution

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🛠️ Fixes #<issue_number_goes_here>

[gemini-code-assist[bot]]

Summary of Changes

Hello @shobsi, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses an inconsistency in how BigQuery numeric types (NUMERIC and BIGNUMERIC) are represented in the output of the bigquery-sql and bigquery-execute-sql tools. By converting these values from rational fractions to decimal strings, the tools' output now accurately matches the expected JSON format of the BigQuery REST API, improving data consistency and interoperability.

Highlights

• BigQuery Numeric Output Formatting: NUMERIC and BIGNUMERIC values from BigQuery tools (bigquery-sql and bigquery-execute-sql) are now formatted as decimal strings (e.g., "9.5") instead of rational fractions (e.g., "19/2"). This ensures the output matches the BigQuery REST API JSON format.
• New NormalizeValue Function: A new utility function, NormalizeValue, has been introduced in internal/tools/bigquery/bigquerycommon to handle the conversion of *big.Rat types to decimal strings with 38-digit precision and trailing zero trimming. It also recursively processes nested slices and maps.
• Tool Integration: Both bigquery-sql and bigquery-execute-sql tools have been updated to utilize the new NormalizeValue function to ensure consistent output formatting for numeric values.
• Comprehensive Testing: Extensive unit tests have been added for the NormalizeValue function in internal/tools/bigquery/bigquerycommon/conversion_test.go to cover various data types and nested structures, ensuring its correctness.

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