Lightweight File Editing Tab with Next Edit Suggestions (NES) for Quick Solution Verification

### Feature summary

Ability to perform lightweight file edits (e.g., tweaking config or .env files) directly within a dedicated tab in the GitHub Copilot app with Next Edit Suggestions (NES) enabled, allowing quick manual verification of suggested solutions without context switching.

### What problem are you trying to solve?

When verifying code changes proposed by GitHub Copilot, we often need to make small adjustments to settings, environment variables, or configuration files to manually test and verify if the proposed solution is correct.

Currently, having to switch out of the Copilot app into a full IDE just to tweak a single key in a .env or config file breaks the feedback loop and introduces friction during testing.

### Proposed solution

Introduce a lightweight, single-file editing tab within the GitHub Copilot app powered by Next Edit Suggestions (NES).

Key capabilities:
* Dedicated File Edit Tab: View and tweak setup, .env, .yaml, or .json files directly in the app.
* NES Integration: Enable Next Edit Suggestions within the edit tab to rapidly complete repetitive config adjustments, parameters, or environment keys.
* Rapid Verification: Adjust settings and immediately verify Copilot-suggested solutions in real-time.

Scope & Boundaries (Non-Goals):
To ensure this does not conflict with full-fledged IDEs like VS Code:
* In Scope: Lightweight single-file inline text edits and NES-driven completion specifically intended for rapid manual verification. Restrict file editing to a single tab to reinforce this behaviour.
* Out of Scope: Multi-file tree navigation, extension ecosystems, language server protocol (LSP) diagnostics, or rich debugging suites.

### Workflow impact

_No response_

### Installation context

_No response_

### Additional context

_No response_

Issue không nêu tên file, test hay entry point hiện có nào. Hãy bắt đầu bằng cách xác định các entry point hiện tại của app cho file-editing và Next Edit Suggestions, sau đó so sánh các khả năng của chúng với phạm vi single-tab đã nêu và các mục tiêu không thuộc phạm vi. Hoàn thành có nghĩa là có single-file edits nhẹ, hỗ trợ NES và xác minh nhanh mà không cần multi-file navigation, LSP diagnostics hoặc gỡ lỗi chuyên sâu.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.