Add smoke tests for opencode-rails

14 tests across 4 files covering the gem's public surface:

  test/opencode/loading_test.rb (5 tests)
    - All 12 gem-provided constants resolve after require
    - Transitively-loaded opencode-ruby constants are present
    - Opencode::Session source_location points at this gem
    - Opencode::Client source_location points at opencode-ruby
    - Opencode::Rails::VERSION matches semver

  test/opencode/error_reporter_test.rb (3 tests)
    - report is no-op without adapter (returns nil)
    - report forwards error + opts to adapter
    - report works with no kwargs

  test/opencode/exchange_test.rb (3 tests)
    - Initializes with empty / nil / Array(coerced) messages
    - tool_artifacts(exclude:) doesn't raise on empty input

  test/opencode/artifact_test.rb (3 tests)
    - Filename/content/content_type/metadata readers
    - metadata defaults to {} when omitted
    - Trust metadata round-trips

Behavioral tests for Session/Turn/MessageArtifacts (AR + ActiveStorage
fixtures) stay in the host application (ajent-rails) where the
integration environment exists. Same pattern as opencode-ruby:
  - gem smoke tests prove load-time correctness
  - host tests prove integration correctness

test/opencode/loading_test.rb

@@ -0,0 +1,54 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+# Smoke test: every constant the gem promises is defined and points at
+# the right kind of object. If require "opencode-rails" loads cleanly,
+# this passes. If the require chain drifts or a file fails to load,
+# this catches it before downstream apps do.
+class Opencode::LoadingTest < Minitest::Test
+  GEM_PROVIDED_CONSTANTS = %w[
+    Session Turn Exchange Artifact Sandbox SandboxFile
+    Transform Impostor MessageArtifacts UploadedFilesPrompt
+    ToolDisplay ErrorReporter
+  ].freeze
+
+  # Reply / Tracer / Client / etc. ship in opencode-ruby and are
+  # transitively required by opencode-rails' umbrella. Verify the
+  # require chain pulled them in.
+  TRANSITIVE_CONSTANTS_FROM_OPENCODE_RUBY = %w[
+    Client Reply ReplyObserver Tracer Prompts
+    ResponseParser ToolPart PartSource Todo
+    Instrumentation Error
+  ].freeze
+
+  def test_gem_provides_expected_constants
+    GEM_PROVIDED_CONSTANTS.each do |name|
+      assert Opencode.const_defined?(name),
+        "Expected Opencode::#{name} to be defined after `require \"opencode-rails\"`"
+    end
+  end
+
+  def test_transitively_loads_opencode_ruby_constants
+    TRANSITIVE_CONSTANTS_FROM_OPENCODE_RUBY.each do |name|
+      assert Opencode.const_defined?(name),
+        "Expected Opencode::#{name} to be defined transitively via opencode-ruby"
+    end
+  end
+
+  def test_session_constant_points_at_this_gem
+    location = Opencode::Session.instance_method(:initialize).source_location.first
+    assert_match %r{/opencode-rails/}, location,
+      "Expected Opencode::Session to be loaded from opencode-rails, got: #{location}"
+  end
+
+  def test_client_constant_points_at_opencode_ruby
+    location = Opencode::Client.instance_method(:initialize).source_location.first
+    assert_match %r{/opencode-ruby/}, location,
+      "Expected Opencode::Client to come from opencode-ruby, got: #{location}"
+  end
+
+  def test_version_constant
+    assert_match(/\A\d+\.\d+\.\d+/, Opencode::Rails::VERSION)
+  end
+end