fix: preserve cursor position in JSON body editor

[tejaswinisa1]

PR Description

This PR fixes an issue in the JSON request body editor where typing in the middle of text could cause characters to appear in incorrect positions (e.g., "answer" becoming "anwe\nr").

The issue was caused by resetting the cursor position to the end on every widget update.

Related Issues

• Fixes fix: preserve cursor position in JSON body editor #1574

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
• I have run the tests (flutter test) and all tests are passing

Added/updated tests?

• Yes
• No, and this is why: This change is limited to UI cursor behavior and does not affect business logic. The issue was verified manually through interaction in the JSON editor.

OS on which you have developed and tested the feature?

• Windows
• macOS
• Linux