fix(timeseries): prevent duplicate dataset creation race condition

[sentry]

This PR addresses the GraphQLError: duplicate key value violates unique constraint "name_repository_id_unique" (API-ES8).

Problem:
When multiple concurrent GraphQL requests, specifically GetReposCoverageMeasurements, attempt to fetch coverage measurements for an owner, a race condition can occur. The owner_coverage_measurements_with_fallback function identifies timeseries_dataset entries that are missing for certain repositories and then attempts to create them using Dataset.objects.bulk_create.

Root Cause:
If two or more requests simultaneously identify the same set of missing datasets and proceed to bulk_create, the second (or subsequent) bulk_create operation will attempt to insert rows that already exist, leading to a UniqueViolation (IntegrityError) on the name_repository_id_unique constraint.

Solution:
Added ignore_conflicts=True to the Dataset.objects.bulk_create call in timeseries/helpers.py. This leverages PostgreSQL's ON CONFLICT DO NOTHING behavior, allowing concurrent inserts to proceed without error if a row with the same unique key already exists. This effectively resolves the race condition.

Impact on trigger_backfill:
Confirmed that trigger_backfill gracefully handles an empty list of created_datasets (which can occur if all datasets were created by a racing process and ignore_conflicts=True was used), ensuring no further issues.

Legal Boilerplate

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Fixes API-ES8