Ensure we send 406 errors for unsupported accept types

Looks like we have bots etc poking around for things, not sure why this got logged into sentry.

For this class of situation we should gracefully return a 406.

Sentry Issue: LIGHTNING-AK

Phoenix.NotAcceptableError: no supported media type in accept header.

Expected one of ["html"] but got the following formats:

  * "application/dns-json" with extensions: []

To accept custom formats, register them under the :mime library
in your config/config.exs file:

    config :mime, :types, %{
      "application/xml" => ["xml"]
    }

And then run `mix deps.clean --build mime` to force it to be recompiled.

  File "lib/phoenix/controller.ex", line 1606, in Phoenix.Controller.refuse/3
  Module "Elixir.LightningWeb.Router", in LightningWeb.Router.browser/2
  File "lib/lightning_web/router.ex", line 1, in LightningWeb.Router.__pipe_through3__/1
  File "lib/phoenix/router.ex", line 475, in Phoenix.Router.__call__/5
  File "lib/lightning_web/endpoint.ex", line 1, in LightningWeb.Endpoint.plug_builder_call/2
...
(3 additional frame(s) were not displayed)

(Phoenix.NotAcceptableError no supported media type in accept header.

Expected one of ["html"] but got the following formats:

  * "application/dns-json" with extensions: []

To accept custom formats, register them under the :mime library
in your config/config.exs file:

    config :mime, :types, %{
      "application/xml" => ["xml"]
    }

And then run `mix deps.clean --build mime` to force it to be recompiled.
)

Start with the browser pipeline in lib/lightning_web/router.ex and follow the request path through lib/lightning_web/endpoint.ex, using the reported Phoenix.NotAcceptableError as the reproduction. Confirm that an unsupported Accept header receives HTTP 406 rather than surfacing the exception, and add or update the relevant request coverage if the repository provides it.