integrate RoBERTa model as the new default

[SarahZahran1]

Description

This PR adds RoBERTa (cardiffnlp/twitter-roberta-base-sentiment-latest) as a new sentiment analysis model alongside the existing BERTweet model, and sets it as the default model.

Changes

• Added app/models/roberta_model.py with the RoBERTaSentiment class
• Updated app/data/sentiment_data.py to support both BERTweet and RoBERTa models
• Updated config.yaml to add RoBERTa configuration and set it as the default model
• Updated README.md to reflect the new model in the folder structure

Motivation

This PR introduces cardiffnlp/twitter-roberta-base-sentiment-latest as the default sentiment analysis model. Testing demonstrates that this RoBERTa-based model provides significantly more accurate results and better nuance in sentiment detection compared to the previous BERTweet implementation, especially for modern social media text and edge cases.

How to switch between models

In config.yaml:

sentiment_analysis:
  default_model: "roberta"  # or "bertweet"

[BasmaElhoseny01]

LGTM i Have Checked it but will test and then we can merge :D

Great work sara

[BasmaElhoseny01]

Revert the change here

[SarahZahran1]

@BasmaElhoseny01 Done