.NET: .NET: fix: Forward x-agent-user-id to toolbox proxy to resolve OAuth consent binding #8379

[sunoj (sun-9545sunoj)]

Motivation & Context

Fixes #8379                                                                                                  
                                                                                                             
When a hosted agent uses a pre-registered toolbox with a per-user OAuth MCP connection, the consent link in  

its oauth_consent_request was being bound to the container's managed identity rather than the calling user.
This caused sign-in to fail with Code <id> not found on the callback.

### Description & Review Guide                                                                               
                                                                                                             
- **What are the major changes?**                                                                            
  - Updated `HostedCallContext` to capture and store the `UserId` (the `UserIdKey` from `PlatformContext`) in

addition to CallId.
- Updated AgentFrameworkResponseHandler to re-apply HostedCallContext.UserId alongside CallId at
yield boundaries, ensuring the user ID is correctly scoped per request.
- Updated FoundryToolboxBearerTokenHandler to forward the x-agent-user-id HTTP header during egress to
the toolbox proxy, mirroring the existing behavior for x-agent-foundry-call-id.
- What is the impact of these changes?
- The toolbox proxy will now resolve the server-side caller context for the human user rather than
defaulting to the container's Azure Managed Identity. This guarantees that the OAuth consent link generated
(CONSENT_REQUIRED) correctly binds to the caller, allowing the sign-in and subsequent tools lookup to succeed.
- What do you want reviewers to focus on?
- Verify that the yield boundaries and egress interception in AgentFrameworkResponseHandler properly
reapply the UserId.

### Related Issue                                                                                            
                                                                                                             
Fixes #8379                                                                                                  
                                                                                                             
### Contribution Checklist                                                                                   
                                                                                                             
- [x] The code builds clean without any errors or warnings                                                   
- [x] All unit tests pass, and I have added new tests where possible                                         
- [x] The PR follows the [Contribution Guidelines](https://github.com/microsoft/agent-                       

framework/blob/main/CONTRIBUTING.md)
- [x] This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
- [x] This is not a breaking change.

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[sunoj (sun-9545sunoj)]

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