test: http_get injection for GefsClient + HrrrClient

- GefsClient (73%): http_get injection for fetch_idx + grib download,
  tests for timeout + HTTP error paths, dewpoint_from_rh edge cases
- HrrrClient: http_get injection for do_fetch_idx + range downloads
- GefsClient fetch_grid_profiles error-path tests with stubbed HTTP

Coverage: 79.68%

lib/microwaveprop/weather/gefs_client.ex

@@ -236,8 +236,13 @@ defmodule Microwaveprop.Weather.GefsClient do
     ":(#{vars}):"
   end
 
+  defp http_get(url, opts) do
+    runner = Application.get_env(:microwaveprop, :gefs_http_get, &Req.get/2)
+    runner.(url, opts)
+  end
+
   defp fetch_idx(url) do
-    case Req.get(url, req_options()) do
+    case http_get(url, req_options()) do
       {:ok, %{status: 200, body: body}} -> {:ok, body}
       {:ok, %{status: status}} -> {:error, "GEFS idx HTTP #{status}"}
       {:error, reason} -> {:error, reason}
@@ -265,7 +270,7 @@ defmodule Microwaveprop.Weather.GefsClient do
       |> Task.Supervisor.async_stream_nolink(
         merged,
         fn {start, stop} ->
-          case Req.get(url, [{:headers, [{"Range", "bytes=#{start}-#{stop}"}]} | req_options()]) do
+          case http_get(url, [{:headers, [{"Range", "bytes=#{start}-#{stop}"}]} | req_options()]) do
             {:ok, %{status: 206, body: body}} -> {:ok, start, body}
             {:ok, %{status: status}} -> {:error, "GEFS grib HTTP #{status}"}
             {:error, reason} -> {:error, reason}

lib/microwaveprop/weather/hrrr_client.ex

@@ -475,13 +475,18 @@ defmodule Microwaveprop.Weather.HrrrClient do
   end
 
   defp do_fetch_idx(url) do
-    case Req.get(url, req_options()) do
+    case http_get(url, req_options()) do
       {:ok, %{status: 200, body: body}} -> {:ok, body}
       {:ok, %{status: status}} -> {:error, "HRRR idx HTTP #{status}"}
       {:error, reason} -> {:error, reason}
     end
   end
 
+  defp http_get(url, opts) do
+    runner = Application.get_env(:microwaveprop, :hrrr_http_get, &Req.get/2)
+    runner.(url, opts)
+  end
+
   @doc """
   Downloads the given byte ranges from a GRIB2 URL and concatenates
   them in offset order. Merges adjacent/overlapping ranges and runs
@@ -583,7 +588,7 @@ defmodule Microwaveprop.Weather.HrrrClient do
       |> Task.Supervisor.async_stream_nolink(
         merged,
         fn {start, stop} ->
-          case Req.get(url, [{:headers, [{"Range", "bytes=#{start}-#{stop}"}]} | req_options()]) do
+          case http_get(url, [{:headers, [{"Range", "bytes=#{start}-#{stop}"}]} | req_options()]) do
             {:ok, %{status: 206, body: body}} -> {:ok, start, body}
             {:ok, %{status: status}} -> {:error, "HRRR grib HTTP #{status}"}
             {:error, reason} -> {:error, reason}

test/microwaveprop/weather/gefs_client_test.exs

@@ -352,4 +352,53 @@ defmodule Microwaveprop.Weather.GefsClientTest do
       end
     end
   end
+
+  describe "dewpoint_from_rh/2 edge cases" do
+    test "handles 0% RH by clamping to a small positive value" do
+      result = GefsClient.dewpoint_from_rh(25.0, 0.0)
+      assert is_float(result)
+      assert result < 25.0
+    end
+
+    test "100% RH at 25°C returns ~25°C dewpoint" do
+      result = GefsClient.dewpoint_from_rh(25.0, 100.0)
+      assert_in_delta result, 25.0, 0.5
+    end
+
+    test "50% RH at 25°C returns dewpoint below 25°C" do
+      result = GefsClient.dewpoint_from_rh(25.0, 50.0)
+      assert result < 25.0
+      assert result > 10.0
+    end
+  end
+
+  describe "fetch_grid_profiles/3 with injected http_get" do
+    setup do
+      prev = Application.get_env(:microwaveprop, :gefs_http_get)
+
+      on_exit(fn ->
+        if prev,
+          do: Application.put_env(:microwaveprop, :gefs_http_get, prev),
+          else: Application.delete_env(:microwaveprop, :gefs_http_get)
+      end)
+
+      :ok
+    end
+
+    test "returns error when idx fetch fails" do
+      Application.put_env(:microwaveprop, :gefs_http_get, fn _url, _opts ->
+        {:error, :timeout}
+      end)
+
+      assert {:error, _} = GefsClient.fetch_grid_profiles(~D[2026-04-18], 12, 0)
+    end
+
+    test "returns error when idx returns non-200" do
+      Application.put_env(:microwaveprop, :gefs_http_get, fn _url, _opts ->
+        {:ok, %{status: 404}}
+      end)
+
+      assert {:error, _} = GefsClient.fetch_grid_profiles(~D[2026-04-18], 12, 0)
+    end
+  end
 end