Setting Module Attributes in endpoint during compile time for Environment (could not fetch application environment)

Hi there Elixir Community,

IN the phoenix Endpoint, I want to have different session options for dev environment and the production environment, like setting cookie secure, etc.

Issue I am facing is when I use Application.get_env or Application.fetch_env it gives warning and CI fails ,
the Warning: is discouraged in the module body, use Application.compile_env/3 instead

When I use Compile env I get this error.

* (ArgumentError) could not fetch application environment [AdminWeb.Endpoint, :session_options] for application :admin because the application was not loaded nor configured
    (elixir 1.14.4) lib/application.ex:602: Application.compile_env!/3
    lib/admin_web/endpoint.ex:5: (module)
    (elixir 1.14.4) lib/kernel/parallel_compiler.ex:340: anonymous fn/5 in Kernel.ParallelCompiler.spawn_workers/7
Elixir

I have two apps in umbrella
AdminWeb and ServiceProviderWeb

Code for AdminWeb ,ServiceProvider

defmodule AdminWeb.Endpoint do
  @moduledoc false
  use Phoenix.Endpoint, otp_app: :admin

  @session_options Application.compile_env!(:admin, [__MODULE__, :session_options])

  socket("/admin/live", Phoenix.LiveView.Socket,
    websocket: [connect_info: [session: @session_options]]
  )
#other lines omitted
  plug(Plug.Session, @session_options)
end

defmodule ServiceProvider.Endpoint do
  @moduledoc false
  use Phoenix.Endpoint, otp_app: :service_provider
  @session_options Application.compile_env!(:service_provider, [__MODULE__, :session_options])


  socket("/service_provider/live", Phoenix.LiveView.Socket,
    websocket: [connect_info: [session: @session_options]]
  )

  plug(Plug.Session, @session_options)

end
 

dev.exs

 
config :admin, AdminWeb.Endpoint,
  server: true,
  http: [port: 4001],
  debug_errors: true,
  code_reloader: true,
  check_origin: false,
  watchers: [
    node: [
      "node_modules/webpack/bin/webpack.js",
      "--mode",
      "development",
      cd: Path.expand("../apps/admin/assets", __DIR__)
    ]
  ],
  live_reload: [
    patterns: [
      ~r"priv/static/.*(js|css|png|jpeg|jpg|gif|svg)$",
      ~r"priv/gettext/.*(po)$",
      ~r"lib/admin_web/.*(ex)$",
      ~r"lib/admin_web/.*(eex)$",
      ~r"lib/admin_web/.*(heex)$"
    ],
    url: "/admin/phoenix/live_reload/socket"
  ],
  session_options: [
    max_age: 120 * 60 * 60,
    store: AdminWeb.Session.Store,
    key: "_a_dev_web_key_",
    signing_salt:  "rand_Contatvalie"
  ]

config :service_provider, ServiceProvider.Endpoint,
  server: true,
  http: [port: 4003],
  debug_errors: true,
  code_reloader: true,
  check_origin: false,
  watchers: [
    node: [
      "node_modules/webpack/bin/webpack.js",
      "--mode",
      "development",
      cd: Path.expand("../apps/service_provider/assets", __DIR__)
    ]
  ],
  live_reload: [
    patterns: [
      ~r"priv/static/.*(js|css|png|jpeg|jpg|gif|svg)$",
      ~r"priv/gettext/.*(po)$",
      ~r"lib/service_provider/.*(ex)$",
      ~r"lib/service_provider/.*(eex)$",
      ~r"lib/service_provider/.*(heex)$"
    ],
    url: "/service_provider/phoenix/live_reload/socket"
  ],
  session_options: [
    store: :cookie,
    key: "_sp_dev_web_key_",
    signing_salt: "rand_Constant tvallue",
    max_age: 120 * 60 * 60
  ]

Prod.exs

config :admin, AdminWeb.Endpoint,
  cache_static_manifest: "priv/static/cache_manifest.json",
  session_options: [
    max_age: 120 * 60 * 60,
    store: AdminWeb.Session.Store,
    key: "_a_web_key_",
    signing_salt: "random_sallt",
    secure: true,
    http_only: true,
    sign: true
  ]

config :service_provider, ServiceProvider.Endpoint,
  cache_static_manifest: "priv/static/cache_manifest.json",
  session_options: [
    store: :cookie,
    key: "_sp_web_key_",
    signing_salt: "random_sallt",
    max_age: 120 * 60 * 60,
    secure: true,
    http_only: true,
    sign: true,encrypt: true
  ]

`My questions`

Q1. What is the solution for it, what mistake I am making?.What wrong settings I have done here?
Q2. Are dev.exs and prod.exs executed once on compilation? so can i use `Base.encode64(:crypto.strong_rand_bytes(64), padding: false)` for `signing_salt`.
Q3. what are the generral best practices tips that you guys can share?