Fix using obsolete WebHostBuilder in wasm DevServer/DebugProxy

[akoeplinger]

It was obsoleted in dotnet/aspnetcore#62785

Switch to new HostBuilder().ConfigureWebHost instead.

[Copilot]

Pull Request Overview

This PR updates the WASM host and debug proxy components to use the modern ASP.NET Core hosting pattern instead of the obsolete WebHostBuilder. The changes replace direct WebHostBuilder instantiation with the newer HostBuilder().ConfigureWebHost() pattern to address API deprecation.

Key changes:

• Migrated from WebHostBuilder to HostBuilder().ConfigureWebHost()
• Updated return types from IWebHost to IHost throughout the affected APIs
• Added necessary using Microsoft.Extensions.Hosting directives

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/mono/wasm/host/WebServer.cs	Updated WebServer startup to use HostBuilder pattern and changed return type to IHost
src/mono/wasm/host/DevServer/DevServer.cs	Updated DevServer startup to use HostBuilder pattern and changed return type to IHost
src/mono/wasm/host/BrowserHost.cs	Added hosting namespace and updated method signatures to use IHost
src/mono/browser/debugger/BrowserDebugHost/DebugProxyHost.cs	Updated debug proxy host creation to use HostBuilder pattern

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