Migrate models to freezed v3

[badnikhil]

PR Description

This PR migrates the core data models to Freezed v3.0, following the new mandatory syntax.
Model Updates: Converted RequestModel, HistoryMetaModel, and HistoryRequestModel to abstract class as required by Freezed v3.

Related Issues

• Closes #

Checklist

• I have gone through the contributing guide
• I have updated my branch and synced it with project main branch before making this PR
• I am using the latest Flutter stable branch (run flutter upgrade and verify)
• [ x I have run the tests (flutter test) and all tests are passing

Added/updated tests?

We encourage you to add relevant test cases.

• Yes
• No, and this is why: not required

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• Windows
• macOS
• Linux

[badnikhil]

@animator
https://pub.dev/packages/freezed/changelog
In v3.0, it is now mandatory to declare @freezed classes as either abstract or sealed.That's Why they are updated.