[OTLP] Improve testing the `add` methods for rules

In the OTLP lib we are not testing the add_promql_path (or logql) methods which wrap around the cos-lib Rules methods:
- https://github.com/canonical/cos-lib/blob/d71b94c3765bbe176b46a05a9bb3ce740272c6bb/src/cosl/rules.py#L485

We should provide some extra utests to assert that the minimal functionality that the wrapper provides is doing what it is supposed to. Specifically the effects on the RuleStore:
- promql
- logql

Start with the OTLP wrapper methods add_promql_path and add_logql, then read the referenced cos-lib src/cosl/rules.py implementation around the Rules methods. Add unit tests that exercise each wrapper and verify its effects on the RuleStore for promql and logql.

Written by the indexing model from the issue text.