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/message_artifacts_test.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+# Contract smoke for Opencode::MessageArtifacts (the idempotent
+# ActiveStorage-backed artifact attachment pipeline). Behavioral
+# coverage — including ActiveStorage attachment, Transform application,
+# error reporting via Opencode::ErrorReporter — lives in the host app.
+class Opencode::MessageArtifactsTest < Minitest::Test
+  def test_initialize_takes_message_feature_and_optional_transforms
+    params = Opencode::MessageArtifacts.instance_method(:initialize).parameters
+    by_kind = params.group_by(&:first).transform_values { |list| list.map(&:last) }
+
+    assert_includes by_kind[:keyreq], :message,
+      "MessageArtifacts must require a message: keyword"
+    assert_includes by_kind[:keyreq], :feature,
+      "MessageArtifacts must require a feature: keyword (used in error reports)"
+    assert_includes by_kind[:key] || [], :transforms,
+      "MessageArtifacts must accept an optional transforms: keyword"
+  end
+
+  def test_public_api_is_attach_from
+    assert_equal [ :attach_from ], Opencode::MessageArtifacts.instance_methods(false),
+      "MessageArtifacts's only public verb is #attach_from"
+  end
+end