Align PeerDAS sampling_size comparison units

[jimmygchen]

The current sampling size logic mixes different units - SAMPLES_PER_SLOT is measured in column samples, while custody_group_count refers to custody groups:

At each slot, a node advertising custody_group_count downloads a minimum of sampling_size = max(SAMPLES_PER_SLOT, custody_group_count) total custody groups.

This discrepancy causes issues when NUMBER_OF_COLUMNS does not exactly match NUMBER_OF_CUSTODY_GROUPS, potentially leading to incorrect sampling_size.

This PR converts the custody_group_count to column count.

[jimmygchen]

I'm not entirely sure if this change is correct, but I think we need to make sure the units used here are consistent.

Alternatively, we may want to change SAMPLES_PER_SLOT to mean "Number of custody groups a node queries per slot" instead - given that the two other custody configs both use custody groups as units:

consensus-specs/specs/fulu/das-core.md

Lines 70 to 74 in 0964db2

	| Name | Value | Description |
	| - | - | - |
	| `SAMPLES_PER_SLOT` | `8` | Number of `DataColumnSidecar` random samples a node queries per slot |
	| `NUMBER_OF_CUSTODY_GROUPS` | `128` | Number of custody groups available for nodes to custody |
	| `CUSTODY_REQUIREMENT` | `4` | Minimum number of custody groups an honest node custodies and serves samples from |

[ppopth]

SAMPLES_PER_SLOT to mean "Number of custody groups a node queries per slot"

The unit of samples should be columns, since we do columns. Think of peer sampling, you sample columns, not custody groups.

[ppopth]

Looks good to me