Enhancement: Clarify to user which URLs are allowed in Viewer pane and which ones are not

System details:

Positron and OS details:

Positron Version: 2025.07.0 (Universal) build 88
Code - OSS Version: 1.100.0
Commit: 27dd8feb5758b5ccecb38b33cbdb0c034c67d786
Date: 2025-06-12T03:34:40.909Z
Electron: 34.5.1
Chromium: 132.0.6834.210
Node.js: 20.19.0
V8: 13.2.152.41-electron.0
OS: Darwin arm64 24.5.0

Interpreter details:

R 4.5.0 and Python 13.3.0

Describe the issue:

User cannot open URLs like https://www.google.com and receives no clear message that this is not allowed.

Steps to reproduce the issue:

1. Type >url in Quick Input Searchbar and pick Viewer: Open URL in Viewe.
2. Type URL. The https://dogfood.team.pct.posit.it/ works, whereas https://google.com does NOT work.
3. In Dev Tools, notice the error message (screenshot in the end).

Expected or desired behavior:

Viewer can open URLs like google.com OR it provides an error message that briefly explains why the URL cannot be open.

Were there any error messages in the UI, Output panel, or Developer Tools console?

• In Dev Tools, yes:
  

• In the actual Viewer tab though, there are no errors, just a blank page:

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Reproduce the issue through Quick Input Searchbar using “Viewer: Open URL in Viewer,” then inspect the Viewer pane and the Dev Tools error for the URL handling entry point. Compare the working Positron URL with google.com and make the Viewer clearly explain rejected URLs or support them; verify the result in the Viewer tab rather than only Dev Tools.