Timestamp Returned In Scientific Notation By API Crashes Field Cast

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### Summary of the issue

**Context**
Write a timestamp field to the BigQuery API that was written using UnixTime stamp from python.
Then try to load it as an object.

**Expected Behavior:**
Timestamp comes back without errors.

**Actual Behavior:**
Getting a TypeError when int tries to parse "1.67899e8".

### API client name and version

google-cloud-bigquery

### Reproduction steps: code

See: [google-cloud-bigquery/google/cloud/bigquery/_helpers.py#L264](https://github.com/googleapis/google-cloud-python/blob/main/packages/google-cloud-bigquery/google/cloud/bigquery/_helpers.py#L264)

Line 264 is: return _datetime_from_microseconds(int(value))

The int cast causes the error when the API returns '1.345345e8', which it does.

### Reproduction steps: supporting files

Write a timestamp to bigqurry using time.time()... its accepted.
Load the value back.... causes error.

### Reproduction steps: actual results

### Reproduction steps: expected results

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### OS & version + platform

_No response_

### Python environment

_No response_

### Python dependencies

_No response_

### Additional context

_No response_