fix fastapi app path to module conversion on Windows (#5858)

### Summary

- addresses #5312
- re-enables `Python - Verify Basic FastAPI App [C903306]` on Windows

### Release Notes

#### New Features

- N/A

#### Bug Fixes

- Fixes bug in file path to module import path conversion in the Python
Extension, which was preventing FastAPI apps from running on Windows
when using the App Launcher (#5312)

### QA Notes

Running
[fastapi_example.py](https://github.com/posit-dev/qa-example-content/blob/main/workspaces/python_apps/fastapi_example/fastapi_example.py)
with the App Launcher should now work on Windows.

#### PR Testing

@:apps

Windows Test Results 🎉 

- test run:
https://github.com/posit-dev/positron/actions/runs/12486724958/job/34847131287

<img width="987" alt="image"
src="https://github.com/user-attachments/assets/ddb9c59a-509c-496e-a7f0-eb961a4359b8"
/>

extensions/positron-python/src/client/positron/webAppCommands.ts

@@ -313,8 +313,7 @@ function pathToModule(p: string): string {
     }
 
     // Otherwise, convert the path to a Python module format.
-    const parts = relativeDir.split(path.sep);
-
+    const parts = relativeDir.split(/[/\\]/);
     return parts.concat(mod).join('.');
 }
 

test/e2e/tests/apps/python-apps.test.ts

@@ -39,7 +39,9 @@ test.describe('Python Applications', {
 		});
 	});
 
-	test('Python - Verify Basic FastAPI App [C903306]', async function ({ app, python }) {
+	test('Python - Verify Basic FastAPI App [C903306]', {
+		tag: [tags.WIN]
+	}, async function ({ app, python }) {
 		const viewer = app.workbench.viewer;
 
 		await app.workbench.quickaccess.openFile(join(app.workspacePathOrFolder, 'workspaces', 'python_apps', 'fastapi_example', 'fastapi_example.py'));