Replace HttpPlugin with HttpServer + HttpServerAdapter

[heyitsaamir]

Summary

• Replace HttpPlugin with HttpServer + HttpServerAdapter pattern — follows the TS PR #433 approach
• HttpServer is no longer a plugin — it's owned directly by App
• Introduces HttpServerAdapter protocol with FastAPIAdapter as the default implementation
• Plugins inject HttpServer via DI and access the framework via server.adapter
• Framework-agnostic handler pattern: async (HttpRequest) -> HttpResponse
• Rewrites remote_function_jwt_middleware to be framework-agnostic
• Adds examples/http-adapters/ with Starlette adapter and non-managed FastAPI examples

Stacked on #249

Test plan

• Run poe test — all tests pass
• Run echo example end-to-end
• Verify DevTools plugin starts correctly alongside main server
• Verify BotBuilder plugin route registration

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