Flow fails to initialize with error "Principal does not have access to API/Operation"

> Please provide us with the following information:
> ---------------------------------------------------------------

### This issue is for a: (mark with an `x`)
```
- [x] bug report -> please search issues before submitting
- [ ] feature request
- [ ] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)
```

### Minimal steps to reproduce
> Follow steps up 'til 4, then run `pf flow test` command.

### Any log messages given by the failure
> As follows:
```
vscode ➜ /workspaces/rag-data-openai-python-promptflow/src (main) $ pf flow test --flow ./copilot_flow --inputs chat_input="how much do the Trailwalker shoes cost?"
Prompt flow service has started...
You can view the trace detail from the following URL:
http://127.0.0.1:23333/v1.0/ui/traces/?#collection=copilot_flow&uiTraceId=0x1d990b212f7a159e901e846e40801416
pf.flow.test failed with UserErrorException: AuthenticationError: Execution failure in 'get_chat_response': (AuthenticationError) Error code: 401 - {'error': {'code': 'PermissionDenied', 'message': 'Principal does not have access to API/Operation.'}}
```

### Expected/desired behavior
> I would expect a 0 exit code and some response from pf. Given the error, I'd like a more descriptive log so I could try debugging the auth issue. I'm not sure where to start.

### OS and Version?
> provided debian 12 dev container running on WSL 2 Docker on Windows 11.

### Versions
> git hash `7e8e6b04727364b2a608fd62b2efa1ffa322d266`

### Mention any other details that might be useful

> ---------------------------------------------------------------
> Thanks! We'll be in touch soon.

Beginne damit, `pf flow test --flow ./copilot_flow --inputs chat_input="how much do the Trailwalker shoes cost?"` zu reproduzieren, und untersuche den im Fehler genannten Ausführungspfad `get_chat_response`. Verfolge die 401-Antwort sowie die konfigurierten Berechtigungen des Principals und der API-Operation; abgeschlossen ist die Aufgabe, wenn entweder der Flow eine Antwort mit Exit-Code 0 zurückgibt oder das Log die Authentifizierungsanforderung eindeutig ausweist.

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