test: add async_http adapter verification tests

Verify async_http adapter is registered, configurable via ENV, and
properly inherits from Faraday::Adapter. Avoids Sync blocks which
deadlock with Minitest's parallel threads.

Author: seuros

test/async_http_adapter_test.rb

@@ -0,0 +1,71 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+# Test that async_http adapter is properly registered and can be configured.
+# Note: Real async HTTP tests with network calls are problematic with VCR/WebMock
+# because Minitest parallel threads conflict with Async's event loop.
+class AsyncHttpAdapterTest < ActiveSupport::TestCase
+  test "async_http adapter is registered with Faraday" do
+    # Verify the adapter is available
+    assert Faraday::Adapter.lookup_middleware(:async_http),
+           "async_http adapter should be registered"
+  end
+
+  test "async_http adapter can be selected via environment variable" do
+    original = ENV["AP_DEFAULT_ADAPTER"]
+    ENV["AP_DEFAULT_ADAPTER"] = "async_http"
+
+    # Build a minimal connection to test adapter selection
+    connection = Faraday.new(url: "https://example.com") do |conn|
+      conn.request :retry
+      conn.response :raise_error
+      default_adapter = ENV.fetch("AP_DEFAULT_ADAPTER", "net_http").to_sym
+      conn.adapter default_adapter
+    end
+
+    # Verify async_http was selected - adapter returns a Handler with klass attribute
+    adapter_handler = connection.builder.adapter
+    assert_equal Async::HTTP::Faraday::Adapter, adapter_handler.klass,
+                 "Connection should use async_http adapter when AP_DEFAULT_ADAPTER is set"
+  ensure
+    if original
+      ENV["AP_DEFAULT_ADAPTER"] = original
+    else
+      ENV.delete("AP_DEFAULT_ADAPTER")
+    end
+  end
+
+  test "default adapter is net_http when env not set" do
+    original = ENV["AP_DEFAULT_ADAPTER"]
+    ENV.delete("AP_DEFAULT_ADAPTER")
+
+    connection = Faraday.new(url: "https://example.com") do |conn|
+      conn.request :retry
+      conn.response :raise_error
+      default_adapter = ENV.fetch("AP_DEFAULT_ADAPTER", "net_http").to_sym
+      conn.adapter default_adapter
+    end
+
+    adapter_handler = connection.builder.adapter
+    assert_equal Faraday::Adapter::NetHttp, adapter_handler.klass,
+                 "Connection should default to net_http when AP_DEFAULT_ADAPTER is not set"
+  ensure
+    ENV["AP_DEFAULT_ADAPTER"] = original if original
+  end
+
+  test "async_http adapter class is properly defined" do
+    # Verify the adapter class exists and has the expected interface
+    adapter_class = Async::HTTP::Faraday::Adapter
+    assert adapter_class < Faraday::Adapter,
+           "Async::HTTP::Faraday::Adapter should inherit from Faraday::Adapter"
+
+    # Verify connection can be built with the adapter (without making requests)
+    connection = Faraday.new(url: "https://example.com") do |conn|
+      conn.adapter :async_http
+    end
+
+    assert_kind_of Faraday::Connection, connection
+    assert_equal Async::HTTP::Faraday::Adapter, connection.builder.adapter.klass
+  end
+end