Fix: Env Variable Text Field Loses Focus After First Character Typing (#1132)

[preetham3127]

PR Description

Fixes an issue where the environment variable text field loses focus immediately after typing the first character.

Changes

• Converted CellField to a StatefulWidget to properly manage input state
• Introduced a persistent TextEditingController
• Preserved cursor position and focus during rebuilds
• Added didUpdateWidget to sync updates without interrupting user input

Result

• Text field retains focus while typing
• Continuous input works as expected without interruption

Scope

• Changes are minimal and limited to field_cell.dart
• No unrelated files were modified
• No additional dependencies introduced

Related Issues

Closes #1132

Checklist

• I have gone through the contributing guide
• I have synced my branch with main
• I have tested the fix locally
• No unrelated changes were made