fix: Unify separators in unique_key construction

[vdusek]

Description

• Use only pipes (|) as separators when constructing unique_key values.
• Since the changes affect only the unique_key of the always_enqueue requests, it is not a breaking change.
• The random string component is placed at the beginning, so an always_enqueue unique key looks like this:

unique_key='7VnDVJgE3TrjbDl86|GET|e3b0c442|e3b0c442|https://crawlee.dev/'

This format is slightly better for debugging, as the individual parts of the unique key (including the URL) are easier to identify.

Issues

• Closes: Unify separators in unique_key construction (| vs _) #1512

Testing

• Current tests pass.

Checklist

• CI passed