fix: re-register after auth server change

[DaleSeo]

Motivation and Context

This is a quick follow-up to #1008. It routed auth/authorization-server-migration through the generic auth flow, which made its dedicated conformance path unreachable. Restoring that path exposed a second issue in #998: after clearing the DCR credentials tied to the previous authorization server, initialize_from_store returned an issuer-mismatch error instead of allowing registration with the new server. This completes the authorization-server migration flow and adds a focused regression test.

How Has This Been Tested?

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

[alexhancock]

Only thing I can see is that now callers can no longer distinguish "AS changed, creds discarded" from "no creds present" as both now return Ok(false) but seems ok