fix #371 use JSON output of gemini cli

[gsehgal]

This PR will address issue 371, where we will use JSON output of Gemini CLI

Test Scenarios:

1. Created an ISSUE which caused failure and verified the output to be in json format:

2. Created a PULL request and verified the positive response was on JSON format.

[MJjainam]

@gemini-cli /review

[gemini-cli]

🤖 Hi @MJjainam, I've received your request, and I'm working on it now! You can track my progress in the logs for more details.

[gemini-cli]

## 📋 Review Summary

This pull request effectively refactors the action to consume the JSON output from the Gemini CLI. This is a significant improvement that allows for more robust parsing of responses and errors.

🔍 General Feedback

• The transition to JSON output is well-implemented, with good use of jq to parse the structured data.
• The fallback to use stderr when JSON is not available or invalid is a good defensive programming practice.
• A few minor suggestions have been made to enhance the shell script's robustness, primarily by replacing echo with safer alternatives in specific contexts.

[gsehgal]

Addressed comments.