[source-square] Incremental cursor rejects timestamps without microseconds

> 🤖 *This issue was generated by an AI Agent.*

## What's missing

Square returns `updated_at` values both with and without fractional seconds, but the connector declares only the microsecond form. Records carrying a whole-second timestamp fail cursor parsing.

## Verified on `master`

`airbyte-integrations/connectors/source-square/manifest.yaml` declares a single cursor format at lines 352-353 and again at 441-442:

```yaml
cursor_datetime_formats:
- "%Y-%m-%dT%H:%M:%S.%fZ"
datetime_format: "%Y-%m-%dT%H:%M:%S.%fZ"
```

There is no `%Y-%m-%dT%H:%M:%SZ` fallback.

**Note:** #68157 (@kdsnunes) is still open and targets exactly this problem. It may be the faster path than a fresh fix.

## Origin

Closed as stale: #68149 (@notoriousblt), open since 2025-10-16 with no author response after review.

Filed to keep a record of the gap. The PR was closed for inactivity, not on merit — credit to the original contributor if this is picked up.

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**Internal Tracking:** https://github.com/airbytehq/oncall/issues/13180