Fix invalid PEP 723 header in new-script template (#1514)

I was reading PEP723 specs trying to understand the scope for supporting
it in envs extension and noticed that the header is invalid in the
template, so sending the PR to fix it.

## Summary
The script template shipped under
`files/templates/new723ScriptTemplate/script.py` opens with `# ///
script_name`, which is **not** a valid PEP 723 inline-script metadata
header — the
[spec](https://packaging.python.org/en/latest/specifications/inline-script-metadata/)
mandates the literal type `script`. Compounding this,
[newScriptProject.ts](src/features/creators/newScriptProject.ts)
substitutes the user's filename into that token at create-time,
producing further invalid headers like `# /// my_script`. This PR
replaces the template's first line with the literal `# /// script` so
the generated script is recognized by every PEP 723–compliant tool, and
adds a unit test that locks the behaviour in.

Author: StellaHuang95

files/templates/new723ScriptTemplate/script.py

@@ -1,4 +1,4 @@
-# /// script_name
+# /// script
 # requires-python = ">=X.XX" TODO: Update this to the minimum Python version you want to support
 # dependencies = [
 #   TODO: Add any dependencies your script requires

src/test/features/creators/newScriptProject.unit.test.ts

@@ -0,0 +1,44 @@
+import assert from 'assert';
+import * as fs from 'fs-extra';
+import * as os from 'os';
+import * as path from 'path';
+
+// Path to the real script template, resolved from the compiled test location
+// (out/test/features/creators/ → workspaceRoot/files/templates/...). We do NOT
+// rely on `NEW_PROJECT_TEMPLATES_FOLDER` because it is anchored at
+// `path.dirname(__dirname)` of the compiled `constants.js`, which resolves to
+// `out/` in test mode and does not contain the bundled template tree.
+const TEMPLATE_PATH = path.join(
+    __dirname,
+    '..',
+    '..',
+    '..',
+    '..',
+    'files',
+    'templates',
+    'new723ScriptTemplate',
+    'script.py',
+);
+
+suite('new723ScriptTemplate / NewScriptProject', () => {
+    let tmpDir: string;
+
+    setup(async () => {
+        tmpDir = await fs.mkdtemp(path.join(os.tmpdir(), 'new-script-test-'));
+    });
+
+    teardown(async () => {
+        await fs.remove(tmpDir);
+    });
+
+    test('Template file starts with a valid PEP 723 header (# /// script)', async () => {
+        const contents = await fs.readFile(TEMPLATE_PATH, 'utf8');
+        const firstNonBlankLine = contents.split(/\r?\n/).find((l) => l.trim().length > 0);
+
+        assert.strictEqual(
+            firstNonBlankLine,
+            '# /// script',
+            'Template must start with a valid PEP 723 `script` header line',
+        );
+    });
+});