Document SignalR authentication refresh (.NET 11)

[BrennanConroy]

.NET 11 adds SignalR authentication refresh, which lets a connected client update its authentication credentials for an active connection without dropping and reconnecting. This PR documents the feature across the SignalR docs.

What's documented

authn-and-authz.md (main feature section, gated to >= aspnetcore-11.0):

• Enable refresh on the server (EnableAuthenticationRefresh, paired with CloseOnAuthenticationExpiration as the safety net that closes connections which aren't refreshed before their token expires).
• Refresh from the .NET client (WithAuthenticationRefresh, RefreshAuthenticationAsync, and the AuthenticationRefreshed / AuthenticationRefreshFailed events).
• Refresh from the JavaScript client (withAuthenticationRefresh, refreshAuthentication, onAuthenticationRefreshed / onAuthenticationRefreshFailed).
• React to a refresh in the hub (OnAuthenticationRefreshedAsync).
• Bidirectional cross-links with the existing "User and role changes during the connection lifetime" section.

configuration.md: Adds the new HttpConnectionDispatcherOptions members (EnableAuthenticationRefresh, MaximumAuthenticationExpiration, OnAuthenticationRefresh) to the advanced HTTP options table.

client-features.md: Adds an "Authentication Refresh" row to the client feature matrix (Server / .NET / JavaScript = 11.0.0; Java and Swift not supported).

Notes for reviewers

• Every API name, signature, default, and behavior was verified against current dotnet/aspnetcore main source rather than the original API proposal. Some behavior changed late: a refresh whose principal maps to a different SignalR user is rejected with HTTP 403 (the connection stays alive with its current user), and a refresh never rekeys Context.UserIdentifier or Clients.User routing. The client callbacks also moved off the options object onto HubConnection events.
• Preview APIs aren't in the API browser yet, so they're written as inline code instead of <xref:> to avoid unresolved-xref build warnings.
• New content is moniker-gated to >= aspnetcore-11.0 using the close/reopen pattern, since Learn monikers can't nest.
• Blazor's ComponentHub override of OnAuthenticationRefreshedAsync is intentionally out of scope here; it belongs in the Blazor docs.

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Internal previews

File	Preview link
aspnetcore/signalr/authn-and-authz.md	aspnetcore/signalr/authn-and-authz
aspnetcore/signalr/client-features.md	aspnetcore/signalr/client-features
aspnetcore/signalr/configuration.md	aspnetcore/signalr/configuration

[wadepickett]

@BrennanConroy: Approved. Looks great, I'm not spotting any issues. I'll put off merging however until the original requested reviewer has a chance to look it over.