feat: add tests and migration docs for using message name for anonymous payload schemas

[Shyam-Raghuwanshi]

Description

• Add tests for issue [FEATURE] Change the name of the generated model class withe name of message name. #1996 covering v2 and v3 named messages with anonymous payloads
• Add migration docs section explaining the naming behavior change in version-5-to-6.md
• Target next branch as this is a breaking change

Related Issue

#1996

Checklist

• The code follows the project's coding standards and is properly linted (npm run lint).
• Tests have been added or updated to cover the changes.
• Documentation has been updated to reflect the changes.
• All tests pass successfully locally.(npm run test).

[netlify]

✅ Deploy Preview for modelina canceled.

Name	Link
🔨 Latest commit	769b876
🔍 Latest deploy log	https://app.netlify.com/projects/modelina/deploys/699b67f971126000086cd163

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud