Bug fix for nft::insert_levels(). All tests pass.

[koniksedy]

This PR includes:

• A bug fix for the function mata::nft::insert_level().
  • The main issue was that the result transducer was constructed with an empty delta (on line 387). Consequently, it did not know which number to choose for the new state created using the method mata::nfa::add_state(). This led to the creation of inner states (states with new levels) with the same names as existing states, thereby causing issues with delta.
• Repairs to the expected transducer (on line 638). This transducer contained END_MARKER, even though these markers were only present at synchronization levels.
• Uncommenting the remaining tests for mata::nft::replace_reluctant_literal(). All tests pass.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 74.16%. Comparing base (793521e) to head (be63c6b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## reluctant_replace #393 +/- ## ===================================================== - Coverage 74.28% 74.16% -0.13% ===================================================== Files 43 43 Lines 5456 5457 +1 Branches 1215 1215 ===================================================== - Hits 4053 4047 -6 - Misses 955 966 +11 + Partials 448 444 -4 ```

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