Fail loudly on null Drivers in DriverRepository

[Copilot]

driverResponse.Drivers ?? [] silently masked mapping/deserialization failures — null Drivers would produce a successful-looking ingestion that wrote nothing.

Changes

• DriverRepository.cs: Replace null-coalescing fallback with explicit null guards; log + throw on null driverResponse (ArgumentNullException) or null Drivers collection (InvalidOperationException), consistent with the pattern in ResultsRepository.cs

// Before
await cosmosDataRepository.UpsertItemsAsync(driverResponse.Drivers ?? []);

// After
if (driverResponse is null)
{
    logger.LogError("driverResponse is null in WriteDriversAsync.");
    throw new ArgumentNullException(nameof(driverResponse));
}

if (driverResponse.Drivers is null)
{
    logger.LogError("driverResponse.Drivers is null in WriteDriversAsync.");
    throw new InvalidOperationException("Driver response contains a null Drivers collection.");
}

await cosmosDataRepository.UpsertItemsAsync(driverResponse.Drivers);

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