Validation fails silently in Anemoi training when time-range does not span the data

[manvendra9099]

Describe the bug
We found that validation doesn't run at all (without any warning/error) when dataloader.validation.start/end dates don't overlap with the actual data in the dataset

Example:

dataloader:
  validation:
    start: 2020-07-01 00:00:00   # <-- worked
    # start: 2021-01-01 00:00:00 # <-- silently skips validation
    end: 2020-07-31 23:59:00 
    # end: 2021-12-31 23:59:00

Run training with this config. The validator does not run and logs no validation metrics or warnings.

Actual behaviour:
Training runs normally
No validation occurs
No warning or error is logged

Expected behavior:
When the validation range yields zero samples: Throw an error or at least a warning early in the dataloader setup
Include in the message:

• requested validation.start/end
• dataset actual coverage (tmin/tmax)

Recommended fix
Raise a ValueError in _dates_to_indices if the requested date range yields zero samples, with a message showing the requested and dataset date ranges.