Add focused smoke tests for the remaining 8 gem classes + adapter-raises edge

Sandi S2 (smoke coverage was 4/12) + Tobi T7 (adapter exception path
untested) consensus actions. 38 new tests across 9 files take total
from 15 -> 53 tests, 50 -> 134 assertions.

Per-class breakdown:

  session_test.rb                (2 tests)
    Contract on initialize parameters (positional record + keyword
    callables) and public surface (ensure!/recreate!/abort!/just_created?).
    AR fixtures stay in the host suite.

  turn_test.rb                   (4 tests)
    Required + optional keyword arg contracts (locks the 9 required
    + 8 optional keys against drift). Public surface = [:call] only.
    Result struct exercised as a value object with status predicates
    and cost/token delegation.

  message_artifacts_test.rb      (2 tests)
    Contract on initialize parameters; public surface = [:attach_from].

  impostor_test.rb               (3 tests)
    Initialize keyword contract; delegation to ActiveStorage attachment
    via a Struct double on #filename.

  sandbox_test.rb                (5 tests)
    Real tmpdir instantiation: #path / #exists? / #files (Enumerator
    when block-less, yields SandboxFile values when files present) /
    #file lookup-by-basename returns nil for missing.

  sandbox_file_test.rb           (4 tests)
    Real tmpdir + file: basic readers, marcel-backed content_type
    detection, #safe? size-cap rejection, #as_artifact identity
    conversion returns Opencode::Artifact.

  transform_test.rb              (8 tests)
    Documents the abstract contract (source_filename / destination_filename /
    render all raise NotImplementedError). A trivial concrete subclass
    inside the test exercises the default implementations of
    #applies_to?, #trusted?, #owned_filenames, and #purge_impostors?
    that delegate to the two abstract filename methods.

  tool_display_test.rb           (5 tests)
    Known tool canonicalization, status predicates (running/completed/
    errored/in_flight/terminal), unknown-tool fallback, nil-part
    tolerance (callers sometimes pass non-tool parts_json entries).

  uploaded_files_prompt_test.rb  (3 tests)
    Initialize keyword contract; #text returns raw content + empty
    sandbox_file_names map when no files attached; public surface check.

Plus 2 new tests in error_reporter_test.rb (C4):
  - test_adapter_exceptions_propagate: adapter that raises must
    propagate, not silently swallow — operators need to know the
    error tracker is broken.
  - test_report_returns_adapter_return_value: report passes through
    the adapter's return value verbatim (Rails.error.report returns
    the error itself; callers can chain).

Faithful to actual implementations: every test was first written from
the API I expected, then corrected against the class internals when
errors surfaced. The corrections themselves document the contract:
SandboxFile expects a String sandbox_prefix with trailing separator
(not a Pathname), Transform's filename methods are abstract not
nil-defaulting, etc.

test/opencode/uploaded_files_prompt_test.rb

@@ -0,0 +1,55 @@
+# frozen_string_literal: true
+
+require "test_helper"
+require "fileutils"
+require "tmpdir"
+
+# Smoke test for Opencode::UploadedFilesPrompt. The sandbox_path:
+# inversion (per D14 in the design doc) means we can exercise the
+# happy path against a tmpdir without needing Rails / AR / ActiveStorage
+# fixtures. Behavioral tests covering ActiveStorage attached_blob
+# enumeration live in the host application.
+class Opencode::UploadedFilesPromptTest < Minitest::Test
+  # Minimal stub for a user message: #content (the raw user text) and
+  # #files (an ActiveStorage-like collection with #attached?). Real
+  # behavior is exercised in the host's test suite.
+  FakeMessage = Struct.new(:content, :files, keyword_init: true)
+  EmptyFiles = Struct.new(:attached) do
+    def attached? = attached
+  end
+
+  def setup
+    @tmpdir = Dir.mktmpdir("opencode-rails-uploaded-prompt-test-")
+  end
+
+  def teardown
+    FileUtils.remove_entry(@tmpdir) if @tmpdir && File.exist?(@tmpdir)
+  end
+
+  def test_initialize_takes_three_required_keywords
+    params = Opencode::UploadedFilesPrompt.instance_method(:initialize).parameters
+    required = params.select { |kind, _| kind == :keyreq }.map(&:last).sort
+
+    assert_equal %i[sandbox_name_for sandbox_path user_message], required,
+      "UploadedFilesPrompt requires user_message:, sandbox_path:, sandbox_name_for:"
+  end
+
+  def test_text_returns_raw_content_when_no_files_attached
+    message = FakeMessage.new(content: "hello world", files: EmptyFiles.new(false))
+    prompt = Opencode::UploadedFilesPrompt.new(
+      user_message:     message,
+      sandbox_path:     @tmpdir,
+      sandbox_name_for: ->(file) { file.filename.to_s }
+    )
+
+    assert_equal "hello world", prompt.text,
+      "No attached files => text is the raw user content"
+    assert_equal({}, prompt.sandbox_file_names,
+      "No attached files => sandbox_file_names map stays empty")
+  end
+
+  def test_public_surface
+    assert_equal %i[sandbox_file_names text].sort,
+      Opencode::UploadedFilesPrompt.instance_methods(false).sort
+  end
+end