Allow Using HttpClient BaseAddres for Authority in DiscoveryCache

[bhazen]

What issue does this PR address?
Adds a new constructor to DiscoveryCache to allow the use of the BaseAddress from an HttpClient as the authority. This also required making the _authority member nullable and adding the conditional to find the appropriate value.

[Copilot]

Pull Request Overview

Adds support for creating a DiscoveryCache without an explicit authority by falling back to the HttpClient.BaseAddress, plus tests for success and failure scenarios.

• Adds a new constructor that omits authority and makes _authority nullable.
• Updates GetResponseAsync to resolve the effective address from either the provided authority or the HttpClient BaseAddress.
• Adds tests for both valid BaseAddress usage and the null-address failure case.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
identity-model/src/IdentityModel/Client/DiscoveryCache.cs	Adds nullable authority support, new constructor, and updated logic to determine discovery address.
identity-model/test/IdentityModel.Tests/DiscoveryCacheTests.cs	Adds tests for new constructor behavior and error condition when neither authority nor BaseAddress is provided.

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[bhazen]

Was not thrilled with needing to do this, but the alternative I saw was changing the type of _getHttpClient to be Func<HttpClient> which would be a breaking change. What do others think?

[josephdecock]

Maybe to be defensive we should detect if the cast fails and throw an exception to say that you have to either use an http client or set the authority?

[bhazen]

Yeah, that's probably better than relying on exception from HttpClientDiscoveryExtensions. Updated to throw that slightly below this line if we can't get the authority from either expected place.