.NET: Python: [Feature]: Authentication support from DevUI to Agents

### Description

DevUI has a [simple kind of authentication mechanism](https://learn.microsoft.com/en-us/agent-framework/devui/security?pivots=programming-language-python) based on a Bearer token to prevent unauthorized people from using it. However, it seems like that authentication is limited to that: allowing people to use DevUI or not.

What I am missing is a possibility of letting DevUI authenticate against the Agents that it calls, so that developers can test their Agents with a user-specific context.

Without this, the following is not possible to be tested by DevUI:
- configuring `WithInMemorySessionStore()` + `UseClaimsBasedSessionIsolation()`
- accessing `httpContextAccessor.HttpContext?.User`, e.g. inside a function tool

Possibly this could be circumvented by abstracting away getting the user information. An alternative is to build a custom agent client app for development.

### Code Sample

```markdown

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### Language/SDK

Both

Start by tracing DevUI's existing Bearer-token flow and the calls it makes to Agents. Compare this with WithInMemorySessionStore(), UseClaimsBasedSessionIsolation(), and httpContextAccessor.HttpContext?.User; done should allow DevUI to pass user-specific authentication context so these scenarios can be tested.

Written by the indexing model from the issue text.