The BigQuery functions SAFE_* do not match the BigQuery specification

The BigQuery dialect does not support DECIMAL values with arbitrary types: it only supports two fixed types: NUMERIC and BIGNUMERIC, both with fixed precision and scale.

The runtime implementation of the SAFE_* functions uses the following helper in SqlFunctions:

  /** Returns whether a BigDecimal value is safe (that is, has not overflowed).
   * According to BigQuery, BigDecimal overflow occurs if the precision is greater
   * than 76 or the scale is greater than 38. */
  private static boolean safeDecimal(BigDecimal b) {
    return b.scale() <= 38 && b.precision() <= 76;
  }

This helper does not handle correctly NUMERIC value, only BIGNUMERIC.

Moreover, all the tests in SqlOperatorTests use a type system which doesn't even support DECIMAL values wider than 38 digits. So a test like the following:

    f.checkNull("safe_add(cast(-9.9e75 as DECIMAL(76, 0)), "
        + "cast(-9.9e75 as DECIMAL(76, 0)))");

cannot even create the expected BigDecimal value correctly.

This surfaced during the attempt to fix CALCITE-6322: once the casts to DECIMAL are implemented, some of these tests break.