Reject empty source public keys file in consolidate command

[cyc60]

Description

An empty --source-public-keys-file passed CLI validation (validate_public_keys_file iterates lines, so a zero-line file is accepted) and loaded as an empty list. Since process() builds ConsolidationKeys only when source_public_keys is truthy, the command silently fell through to the auto-pick (ConsolidationSelector) path — ignoring the user's explicit --target-public-key and consolidating validators the user never chose, with no warning.

The consolidate command now raises a clean error when the provided source keys file yields no keys. Files with blank lines were already rejected upfront by the CLI callback, so the check targets the zero-keys case specifically. validate_public_keys_file is left unchanged, as it is shared with --exclude-public-keys-file, where an empty file is harmless.

Adds the first tests for the consolidate command (src/validators/commands/tests/test_consolidate.py).