LLM and test refactor

[clavedeluna]

Overview

This PR moves LLM initialization into its own module and updates tests for easier api

Description

• We want to be able to initialize an LLM client in testing, outside of codemodder context, so moving it out to its own module so it can be used
• updated tests to create a DiffError class so we can differentiate (and catch) code errors versus codemodder diff output errors. This will help LLM codemods mostly.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[clavedeluna]

@drdavella this will need to be released

[drdavella]

@clavedeluna thanks! I'll create a release once this and #622 are merged.