viewer: clicking on open content in editor tab should focus to editor tab if already open

[midleman]

System details:

Positron and OS details:

Positron Version: 2024.12.0 (Universal) build 80
Code - OSS Version: 1.93.0
Commit: c3c4896
Date: 2024-11-26T02:46:47.938Z
Electron: 30.4.0
Chromium: 124.0.6367.243
Node.js: 20.15.1
V8: 12.4.254.20-electron.0
OS: Darwin arm64 23.6.0

Describe the issue:

The Viewer should behave similarly to the Data Explorer and Plots when they are opened in the editor area. Repeatedly clicking on it should focus the existing tab rather than opening a new one.

Steps to reproduce the issue:

1. Right-click on any HTML and select Open in Viewer.
2. Verify that the Viewer displays as expected — PASS.
3. Click the Open the Content in the Editor button.
4. Verify that the content opens in the editor — PASS.
5. Click the Open the Content in the Editor button again.
6. Verify that another editor tab does NOT open — FAIL.

Expected behavior:

If the content is already displayed in the editor, clicking the button should switch focus to the existing tab instead of opening a new one.