fix: Unify separators in unique_key construction (#1569)

### 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: #1512

### Testing

- Current tests pass.

Author: vdusek

src/crawlee/_request.py

@@ -299,7 +299,7 @@ def from_url(
         )
 
         if always_enqueue:
-            unique_key = f'{unique_key}_{crypto_random_object_id()}'
+            unique_key = f'{crypto_random_object_id()}|{unique_key}'
 
         request = cls(
             url=url,