Reshape BrowserOptions server-to-client configuration API per review

[Copilot]

Applies the approved API review decisions for the Blazor BrowserOptions surface that flows host configuration to the browser client via the <!--Blazor-Configuration:{...}--> DOM comment. The wire format is preserved through JSON converters/attributes, so no JS changes are required.

Description

• Render-mode property renames on BrowserOptions (get-only, pre-initialized), with [JsonPropertyName] keeping the existing wire keys:
  • Server → InteractiveServer (server)
  • Ssr → StaticServer (ssr)
  • WebAssembly → InteractiveWebAssembly (webAssembly)
• Type renames: SsrBrowserOptions → StaticServerBrowserOptions, WebAssemblyBrowserOptions → InteractiveWebAssemblyBrowserOptions (InteractiveServerBrowserOptions unchanged).
• Accessor moved off HttpContext: removed BrowserOptionsHttpContextExtensions; GetBrowserOptions is now a static method BrowserOptions.GetBrowserOptions(HttpContext).
• ConfigureBrowser: the HttpContext cascading parameter is now internal, removing it from the public API while remaining functional.
• Updated internal consumers (EndpointHtmlRenderer, Server.AutoPause, test server startup, Root.razor) and PublicAPI.Unshipped.txt.
• Added BrowserOptionsTest covering wire-key preservation, TimeSpan→ms conversion, negated PreserveDom→disableDomPreservation, extension-data flattening, GetBrowserOptions caching/metadata seeding, and MergeInto semantics.

app.MapRazorComponents<App>()
    .WithBrowserOptions(options =>
    {
        options.LogLevel = LogLevel.Warning;
        options.InteractiveServer.ReconnectionMaxRetries = 10;
        options.InteractiveServer.ReconnectionRetryInterval = TimeSpan.FromSeconds(1.5);
        options.StaticServer.PreserveDom = true;
        options.InteractiveWebAssembly.EnvironmentName = "Staging";
        options.InteractiveWebAssembly.EnvironmentVariables["OTEL_ENDPOINT"] = "https://...";
    });