validate_target_id_exists breaks with base URL path prefix

[adamtheturtle]

Problem

When using MockVWS with a base_vws_url that includes a path prefix (e.g. http://example.com/prefix), any request to the VWS API fails with UnknownTarget, even for endpoints that have no target ID in their path.

Root cause

validate_target_id_exists in _services_validators/target_validators.py uses the number of path segments to determine whether a target ID is present:

```python
split_path = request_path.split(sep="/")
request_path_no_target_id_length = 2
if len(split_path) == request_path_no_target_id_length:
return
```

For /targets this gives ["", "targets"] → length 2 → returns early (correct).

But with a path prefix the request path is /prefix/targets, which gives ["", "prefix", "targets"] → length 3 → does not return early → split_path[-1] = "targets" is treated as a target ID → UnknownTargetError.

Reproduction

```python
from mock_vws import MockVWS
from mock_vws.database import CloudDatabase
from vws import VWS

base_vws_url = "http://example.com/prefix"
with MockVWS(base_vws_url=base_vws_url) as mock:
database = CloudDatabase()
mock.add_cloud_database(cloud_database=database)
vws_client = VWS(
server_access_key=database.server_access_key,
server_secret_key=database.server_secret_key,
base_vws_url=base_vws_url,
)
vws_client.list_targets() # raises UnknownTargetError: target ID "targets" does not exist
```

Notes

This is analogous to the urljoin routing fix in #2994 / release 2026.02.22.1 — that fixed the routing layer, but the validators still assume no path prefix.

The same pattern likely affects other validators that parse request_path by segment index (e.g. those in key_validators.py, json_validators.py, name_validators.py) depending on what path-structure logic they contain.