[invoke/verifyState] Add optional `accountLinkingUrl` return type for invoke

[aacebo]

To support a change in the Teams FE, we need to optionally allow developers to configure a static account linking url that, if present, would be returned to the Teams Platform after processing of the activity.

Platform Code:
https://skype.visualstudio.com/SCC/_git/async_messaging_botapiservice/pullrequest/1264762?_a=files#:~:text=var%20obj%20%3D%20JToken,string)obj%3F%5B%22accountLinkingUrl%22%5D%3B

Response Type:
https://github.com/microsoft/teams.net/blob/main/Libraries/Microsoft.Teams.Apps/Response.cs

Sample:

new App(new()
{
    OAuth: new()
    {
        AccountLinkingUrl: '...'
    }
});

[rajan-chari]

PR Ready for Merge

PR #188 implementing this feature has been updated and is ready for review.

What Was Done

✅ Resolved all merge conflicts
✅ Rebased on latest main branch
✅ All 561 tests passing
✅ Build succeeds with no errors

Implementation

The PR adds:

1. AccountLinkingUrl property to OAuthSettings - allows developers to configure a static account linking URL
2. Response body update - when AccountLinkingUrl is configured, the invoke/verifyState handler returns it in the response
3. Test coverage - comprehensive tests validating the functionality

Usage Example

var app = new App(new()
{
    OAuth = new()
    {
        AccountLinkingUrl = "https://example.com/link-account"
    }
});

When a verifyState invoke is received, the response will include:

{
  "accountLinkingUrl": "https://example.com/link-account"
}

cc: Lily Du (@lilyydu) Aamir Jawaid (@heyitsaamir) for review and merge