NES: Update enablement logic (#13714)

Closes #13541.

Also moves a couple of settings into `config.ts` so everything to do
with reading settings is in this one file.

### Validation Steps

1. Disable copilot completions by setting `"github.copilot.enable":
{"*": false}`.
2. Ensure you are logged in to Posit AI.
3. Remove `positron.assistant.aiExcludes` and
`nextEditSuggestions.enable` from your settings.
4. Ensure Posit AI completions are working in an e.g. `.R` file.
5a. Set `"nextEditSuggestions.enable": { "*": false }`. Confirm
completions stop.
5b. Set `"positron.assistant.aiExcludes": [ "*.R" ]`. Confirm
completions stop.
6. Sign out of Posit AI, confirm that when Posit AI is not available,
the language model is stopped. The logging message you are looking for
is "[info] Stopping language server due to logout."

Use the NES output panel in debug mode to see (hopefully) useful
logging:
<img width="1327" height="476" alt="Screenshot 2026-05-22 at 10 39 27"
src="https://github.com/user-attachments/assets/f8b0da27-b343-4560-8bf4-d5a13c3f298b"
/>

---------

Co-authored-by: sharon wang <sharon-wang@users.noreply.github.com>

Author: georgestagg

.vscode-test.js

@@ -73,6 +73,11 @@ const extensions = [
 		workspaceFolder: 'extensions/positron-assistant/test-workspace',
 		mocha: { timeout: 60_000 }
 	},
+	{
+		label: 'next-edit-suggestions',
+		workspaceFolder: path.join(os.tmpdir(), `next-edit-suggestions-${Math.floor(Math.random() * 100000)}`),
+		mocha: { timeout: 60_000 }
+	},
 	{
 		label: 'positron-code-cells',
 		workspaceFolder: path.join(os.tmpdir(), `positron-code-cells-${Math.floor(Math.random() * 100000)}`),