Add Suave-style cookie session primitives to the ASP.NET Core Suave adapter

[Copilot]

Suave-style cookie session flows were missing from the adapter, making it hard to port patterns that rely on statefulForSession and HttpContext.state. This change adds minimal session/cookie primitives aligned with the existing compositional WebPart API.

• Session/cookie API surface

  • Added Filters.statefulForSession to gate routes on session availability.
  • Added HttpContext.state to retrieve ISession from the current request context.
  • Added Session.tryGet / Session.set helpers for typed session key access.
  • Added Request.Cookie.tryGet for cookie reads from HttpRequest.

• Behavioral alignment

  • Session availability is now represented as Some/None in Suave-style flow composition instead of throwing when session middleware is absent.
  • Enables the canonical “counter in session cookie” pattern with existing >=> composition.

• Example usage

  Filters.path "/session"
  >=> Filters.statefulForSession
  >=> (fun ctx ->
    match FSharpPlus.AspNetCore.Suave.HttpContext.state ctx with
    | Some store ->
      let current =
        store
        |> FSharpPlus.AspNetCore.Suave.Session.tryGet "counter"
        |> Option.defaultValue "0"
        |> int
      store |> FSharpPlus.AspNetCore.Suave.Session.set "counter" (string (current + 1))
      Successful.OK (sprintf "Hello %d time(s)" (current + 1)) ctx
    | None ->
      Successful.OK "No session available" ctx)