FE: Messages: Validate JSONPath syntax and show inline error instead of alert

[AmandaBalderas20]

Changes
Added validation for JSONPath syntax in the Preview Modal.
When a user enters an invalid JSONPath expression, instead of showing a browser alert, an inline red error message now appears below the input.
This prevents invalid JSONPath expressions from being stored in localStorage, which previously caused UI reload errors and 404 responses.

Things to focus on
Please verify that:

• Invalid JSONPath inputs correctly trigger the red “Invalid JSONPath syntax” message.
• No browser alerts or console errors appear.
• Valid JSONPath expressions are saved and loaded correctly after reload.

How Has This Been Tested?

• Manually (please, describe, if necessary)
  • Steps:
    1. Open Topics → Messages → Preview.
    2. Enter:
       • Field: assignment
       • Json path: @.name=".*odometer.*" (invalid)
    3. Confirm red message “Invalid JSONPath syntax” appears.
    4. Enter valid JSONPath: $.members[*].client_id → saves correctly.
    5. Reload page — no 404 or console errors.

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings (e.g. Sonar is happy)
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

[github-actions]

Hi AmandaBalderas20! 👋

Welcome, and thank you for opening your first PR in the repo!

Please wait for triaging by our maintainers.

Please take a look at our contributing guide.

[Haarolean]

Hi @AmandaBalderas20, thanks for a contribution. Could you please share a screen recording and/or a screenshot demonstrating the things changed? This would speed up the review, thanks!

[AmandaBalderas20]

Hi! Here’s the screen recording showing the fix in action. Let me know if you’d like me to include anything else!
https://github.com/user-attachments/assets/5b3df985-1404-46d7-8331-6ca0dfd2db3a