fix(session): pass plot backend to attach script

The attach script for user-created R sessions now passes use_httpgd,
use_jgd, and JGD_SOCKET to sess::connect(), matching the managed
terminal behavior. Also fixes terminal test to handle the new
SESS_PLOT_BACKEND env var and config default resolution.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: grantmcdermott

src/test/suite/terminal.test.ts

@@ -23,9 +23,9 @@ suite('R Terminal', () => {
     });
 
     test('makeTerminalOptions sets session watcher environment variables', async () => {
-        // Stub config to enable sessionWatcher
+        // Stub config to enable sessionWatcher with httpgd backend
         const configStub = {
-            get: (key: string) => {
+            get: (key: string, defaultValue?: unknown) => {
                 if (key === 'sessionWatcher') {
                     return true;
                 }
@@ -38,7 +38,7 @@ suite('R Terminal', () => {
                 if (key === 'rterm.option') {
                     return ['--no-save'];
                 }
-                return undefined;
+                return defaultValue;
             }
         };
         sandbox.stub(util, 'config').returns(configStub as unknown as vscode.WorkspaceConfiguration);
@@ -52,6 +52,7 @@ suite('R Terminal', () => {
         assert.ok(options.env['SESS_PIPE']);
         assert.strictEqual(options.env['SESS_RSTUDIOAPI'], 'TRUE');
         assert.strictEqual(options.env['SESS_USE_HTTPGD'], 'TRUE');
+        assert.strictEqual(options.env['SESS_PLOT_BACKEND'], 'httpgd');
         assert.ok(options.env['R_PROFILE_USER']);
         assert.ok(options.env['R_PROFILE_USER'].endsWith(path.join('R', 'profile.R')));
     });