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/tool_display_test.rb

@@ -0,0 +1,55 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+# Smoke tests for Opencode::ToolDisplay — the view-model that converts
+# raw tool-part hashes into Turbo-Stream-friendly props. We exercise
+# the predicate surface for the canonical 'read' tool plus the
+# unknown-tool fallback. Exhaustive per-tool render tests live in the
+# host where the renderer templates are exercised.
+class Opencode::ToolDisplayTest < Minitest::Test
+  def test_known_read_tool_canonicalization
+    display = Opencode::ToolDisplay.new(
+      "type" => "tool", "tool" => "read", "status" => "completed",
+      "input" => { "filePath" => "/sandbox/notes.md" }
+    )
+
+    assert_equal "read", display.canonical_tool
+    assert display.known?
+    assert display.completed?
+    assert display.terminal?
+    refute display.errored?
+    refute display.in_flight?
+  end
+
+  def test_running_status
+    display = Opencode::ToolDisplay.new("type" => "tool", "tool" => "read", "status" => "running")
+    assert display.in_flight?
+    refute display.terminal?
+    refute display.completed?
+  end
+
+  def test_errored_status
+    display = Opencode::ToolDisplay.new(
+      "type" => "tool", "tool" => "edit", "status" => "error", "error" => "permission denied"
+    )
+    assert display.errored?
+    assert display.terminal?
+    refute display.completed?
+  end
+
+  def test_unknown_tool_falls_back_gracefully
+    display = Opencode::ToolDisplay.new("type" => "tool", "tool" => "wat", "status" => "completed")
+    refute display.known?,
+      "Unknown tools must not claim to be known — host renderer dispatches a fallback view"
+    refute_nil display.canonical_tool,
+      "Unknown tools still need a canonical_tool so DOM ids stay stable"
+  end
+
+  def test_nil_part_initializes_safely
+    # ToolDisplay tolerates a nil part because callers sometimes pass
+    # message.parts_json entries that aren't tool parts.
+    display = Opencode::ToolDisplay.new(nil)
+    refute display.known?
+  end
+end