fix(google-api-core): preserve exact timestamp seconds

[rioyu123]

Fixes #18561.

DatetimeWithNanoseconds.timestamp_pb() can shift a historical timestamp forward by one second. For example, 1969-12-31T23:59:59.123456789Z becomes 1970-01-01T00:00:00.123456789Z. The same conversion can round the maximum valid timestamp past protobuf's supported range.

The cause is int(delta.total_seconds()): negative fractions truncate toward zero, and large values lose precision in the floating-point conversion. This change computes whole seconds directly from the timedelta's integer days and seconds. Nanosecond handling, the microsecond fallback and the return type stay unchanged.

Tests

• Added 15 cases checked against protobuf's own conversion methods: historical dates, the maximum timestamp, naive and timezone-aware inputs, and unaffected controls. Nine fail on the base commit.
• Complete google-api-core unit and asyncio suites: 1,504 passed, 2 skipped on Python 3.12.3 with protobuf 7.36.2.
• The affected helper retains 100% statement and branch coverage. Single-environment package coverage is unchanged at 99.67%.
• Pinned Ruff import/format checks and Flake8 pass.

The full Python/dependency matrix and credentialed Cloud system tests were not run.

Checklist

• Opened an issue before writing the code.
• Tests and linter pass in the environment described above.
• Code coverage does not decrease.
• Checked documentation needs: no API or documented behavior change.

[gemini-code-assist]

Code Review

This pull request updates the timestamp_pb method in datetime_helpers.py to calculate seconds using delta.days * 86400 + delta.seconds instead of int(delta.total_seconds()), preventing potential precision loss. Additionally, comprehensive unit tests have been added to verify that whole seconds are preserved correctly across various dates (including pre-1970 and far-future timestamps) and timezone offsets. There are no review comments to address, and the changes look solid.