Mocha test flake: `docker/containers.ts waitForServiceHealthCheck`: `should retry failed requests and eventually succeed`

Successful test:
```
docker/containers.ts waitForServiceHealthCheck
2025-10-21T18:33:07.714Z [info] [docker.containers] Starting TestService health check at http://localhost:9090/health
2025-10-21T18:33:07.715Z [info] [docker.containers] TestService health check request failed: Connection refused, retrying...
2025-10-21T18:33:08.715Z [info] [docker.containers] TestService health check succeeded
✔ should retry failed requests and eventually succeed (1002ms)
```

Failed test:
```
docker/containers.ts waitForServiceHealthCheck
2025-10-21T18:25:32.462Z [info] [docker.containers] Starting TestService health check at http://localhost:9090/health
2025-10-21T18:25:32.463Z [info] [docker.containers] TestService health check request failed: Connection refused, retrying...
2025-10-21T18:25:32.465Z [warning] [viewProviders.resources] No connection row found for connectionId { connectionId: 'vscode-local-connection' }
2025-10-21T18:25:32.536Z [error] [docker.eventListener] stream from event response is null
2025-10-21T18:25:33.464Z [info] [docker.containers] TestService health check request failed: Cannot read properties of undefined (reading 'ok'), retrying...
2025-10-21T18:25:33.552Z [error] [errors] Error: docker ping --> ResponseError: Response returned an error code {
errorType: 'ResponseError',
errorMessage: 'Response returned an error code',
errorStack: 'ResponseError: Response returned an error code\n' +
'\tat SystemApi.request (/Users/dshoup/Dev/confluent_dev/vscode/out/src/clients/docker/runtime.js:143:11)\n' +
'\tat async SystemApi.systemPingRaw (/Users/dshoup/Dev/confluent_dev/vscode/out/src/clients/docker/apis/SystemApi.js:146:22)\n' +
'\tat async SystemApi.systemPing (/Users/dshoup/Dev/confluent_dev/vscode/out/src/clients/docker/apis/SystemApi.js:166:22)\n' +
'\tat async Object.isDockerAvailable (/Users/dshoup/Dev/confluent_dev/vscode/out/src/docker/configs.js:45:18)\n' +
'\tat async _EventListener.listenForEvents (/Users/dshoup/Dev/confluent_dev/vscode/out/src/docker/eventListener.js:81:28)'
}
2025-10-21T18:25:33.552Z [warning] [viewProviders.resources] No connection row found for connectionId { connectionId: 'vscode-local-connection' }
2025-10-21T18:25:34.469Z [info] [docker.containers] TestService health check request failed: Cannot read properties of undefined (reading 'ok'), retrying...
2025-10-21T18:25:35.474Z [info] [docker.containers] TestService health check request failed: Cannot read properties of undefined (reading 'ok'), retrying...
2025-10-21T18:25:36.476Z [info] [docker.containers] TestService health check request failed: Cannot read properties of undefined (reading 'ok'), retrying...
2025-10-21T18:25:37.479Z [warning] [docker.containers] TestService health check timed out after 5s
1) should retry failed requests and eventually succeed
0 passing (8s)
1 failing
1) docker/containers.ts waitForServiceHealthCheck
should retry failed requests and eventually succeed:

AssertionError [ERR_ASSERTION]: Health check should eventually succeed

false !== true

+ expected - actual

-false
+true

at Context. (out/src/docker/containers.test.js:216:23)
```

Theory: because of the fact that `waitForServiceHealthCheck` uses `globalThis.fetch`, and the test is stubbing it and not using sinon [Fake timers](https://sinonjs.org/releases/latest/fake-timers/), I think some other fetch is happening during the actual 1sec backoff before the second attempt.
- failed test case:
```
fetch call 1: TestService health check failed, retrying
fetch call 2: unrelated background fetch request, { ok: true, status: 200 } returned
fetch call 3: TestService doesn't have any stubbed responses to work off of, so response is undefined and failures ensue
(continue failing)
```
- successful test case:
```
fetch call 1: TestService health check failed, retrying
fetch call 2: TestService health check happy
fetch call 3: unrelated background fetch request, normal behavior maybe
```

Proposed fix:
- make a small utility function like `containerFetch()` in a standalone file in https://github.com/confluentinc/vscode/tree/main/src/docker that effectively wraps `fetch` and replace the current fetch behavior: https://github.com/confluentinc/vscode/blob/1629eceb289e5fb36476e9768ded69a078ea45a0/src/docker/containers.ts#L160-L163
- update `fetchStub` to stub that new function instead of `globalThis.fetch` https://github.com/confluentinc/vscode/blob/1629eceb289e5fb36476e9768ded69a078ea45a0/src/docker/containers.test.ts#L227
- bonus: use fake timers instead of waiting the ~1sec for the retry

(This is similar to the [fsWrappers.ts](https://github.com/confluentinc/vscode/blob/main/src/utils/fsWrappers.ts) approach, but different intent since the `fs` functions just can't be stubbed directly. In this case we're just guarding against other fetch calls happening in unrelated parts of the codebase.)