Added json validation

[Aman071106]

PR Description

This PR adds robust JSON validation for the request body editor. It includes:

1. Duplicate Key Detection: Identifies keys that appear multiple times in the same object scope (e.g., {"a": 1, "a": 2}). standard jsonDecode does not catch this, so a custom JsonUtils parser was implemented.
2. Precise Error Reporting: Displays error messages with exact Line and Column numbers.
3. Error Highlighting:
   • Adds a Status Card at the bottom of the editor when JSON is invalid.
   • Hover over the card to view full error details.
   • Tap the card to automatically select and highlight the error in the editor (jumps to the duplicate key or syntax error location).
4. Debouncing: Validation runs 500ms after user stops typing to improve performance.

Related Issues

• Closes Add validation for JSON request body #583

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)
• 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: please replace this line with details on why tests have not been included

Details:

• Added test/utils/json_utils_test.dart to verify duplicate key detection, line counting, and offset calculation logic.
• Manual verification steps included in walkthrough.md.

OS on which you have developed and tested the feature?

• Windows
• macOS
• Linux

Example video

final_json_val_ex.mp4

[rakshaak29]

I tested the current behavior on macOS.
When POST body contains only whitespace, the request is sent and results in a 500 error.
This PR looks like it directly improves that UX by validating JSON before sending.
Happy to test this change locally if needed.

[Aman071106]

@rakshaak29 Thanks for the review. Can you share a video?

[rakshaak29]

Thanks for the response!
I’ve attached a short screen recording demonstrating the behavior when
sending a POST request with invalid JSON in the body.
https://github.com/user-attachments/assets/bc4239d1-d5a7-4b35-954d-47dce7c720c9
Please let me know if you’d like me to test any other scenario.

[Aman071106]

@rakshaak29 Thanks for the video , it would be extremely helpful for merging the pr, if you could add a after video also. This will confirm behaviour on macOS also.
@ashitaprasad @animator Please review this PR as changes are ready .