fix(bigquery): Fix timestamp scientific notation handling in _helpers.py

[pkenjora]

Casting to float first handles scientific notation returned by API. Otherwise its a noop.

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Fixes #17277🦕

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[gemini-code-assist]

Code Review

This pull request modifies the timestamp conversion helper in _helpers.py to cast values to float before converting them to integers. The reviewer noted that casting all values to float can cause precision loss for large integer timestamps (beyond the year 2285) and suggested attempting to parse as an integer first, falling back to float only if a ValueError occurs.

[gemini-code-assist]

Directly casting all values to float before converting to int can lead to precision loss for very large integer values (specifically, integers greater than 2^53 - 1, which corresponds to timestamps beyond the year 2285). Since BigQuery supports timestamps up to the year 9999, we should attempt to parse the value as an integer first to preserve full precision, and only fall back to float parsing if a ValueError occurs (e.g., when scientific notation or a decimal point is present).

            try:
                microseconds = int(value)
            except ValueError:
                microseconds = int(float(value))
            return _datetime_from_microseconds(microseconds)

[pkenjora]

Sure.

[sycai]

Hello @pkenjora ! Could you also provide test coverage for this change? Thanks a lot!