chore(delta): address review suggestions — error handling, dead code, style

Follow-up to 3c178e1 applying the remaining suggestions from the
/review pass:

* Codec.decode_delta/1: replace Jason.decode!/2 with a case on
  Jason.decode/2 that wraps malformed input in ArgumentError and
  rejects non-object top-level JSON. Adds tests for both cases.
* Reducer.state_to_initial_delta/1: remove. Dead code — callers use
  MigrationGenerator.initial_operations_for_state/1 directly.
* do_fetch_operations/4: use Reducer.empty_state/1 instead of the
  third inline copy of the baseline state map, so the empty-state
  schema has one source of truth.
* Mix.Tasks.AshPostgres.SquashSnapshots.run/1: add missing
  @impl Mix.Task.
* apply_squash(..., :full): drop the `_ = folder` suppressor and
  destructure as `_folder` instead.
* Codec.maybe_to_atom/1: add `sobelow_skip ["DOS.StringToAtom"]`
  pragma, matching the convention already used throughout the
  codebase (operation.ex, custom_index.ex, resource_generator/spec.ex).

Author: barnabasJ

lib/migration_generator/migration_generator.ex

@@ -2352,23 +2352,16 @@ defmodule AshPostgres.MigrationGenerator do
 
   defp do_fetch_operations(snapshot, nil, opts, acc) do
     if resource_has_meaningful_content?(snapshot) do
-      empty_snapshot = %{
-        attributes: [],
-        identities: [],
-        schema: nil,
-        custom_indexes: [],
-        custom_statements: [],
-        check_constraints: [],
-        table: snapshot.table,
-        repo: snapshot.repo,
-        base_filter: nil,
-        empty?: true,
-        multitenancy: %{
-          attribute: nil,
-          strategy: nil,
-          global: nil
-        }
-      }
+      # Use the Reducer's canonical empty state so this and delta reduction
+      # share one definition of "empty". `empty_state/1` produces a schema-less
+      # baseline with all the scalar-fields (base_filter, create_table_options,
+      # …) explicitly defaulted.
+      empty_snapshot =
+        AshPostgres.MigrationGenerator.Reducer.empty_state(%{
+          table: snapshot.table,
+          schema: nil,
+          repo: snapshot.repo
+        })
 
       do_fetch_operations(snapshot, empty_snapshot, opts, [
         %Operation.CreateTable{

lib/migration_generator/operation/codec.ex

@@ -159,11 +159,27 @@ defmodule AshPostgres.MigrationGenerator.Operation.Codec do
   `:previous_hash`, `:resulting_hash`, `:migration`, `:generated_at`,
   `:operations` (list of operation structs).
 
-  Raises `ArgumentError` if the version field is missing or not equal to 2 —
-  legacy full-state files must be migrated via `mix ash_postgres.migrate_snapshots`.
+  Raises `ArgumentError` if:
+    * the JSON is malformed
+    * the top-level value is not a JSON object
+    * the version field is missing or not equal to #{@delta_version}
+      — legacy full-state files must be migrated via
+      `mix ash_postgres.migrate_snapshots`.
   """
   def decode_delta(json) when is_binary(json) do
-    decoded = Jason.decode!(json, keys: :atoms!)
+    decoded =
+      case Jason.decode(json, keys: :atoms!) do
+        {:ok, map} when is_map(map) ->
+          map
+
+        {:ok, other} ->
+          raise ArgumentError,
+                "Expected delta snapshot to be a JSON object, got #{inspect(other, limit: :infinity, printable_limit: 200)}"
+
+        {:error, %Jason.DecodeError{} = err} ->
+          raise ArgumentError,
+                "Could not parse delta snapshot JSON: #{Exception.message(err)}"
+      end
 
     case Map.get(decoded, :version) do
       @delta_version ->
@@ -960,6 +976,9 @@ defmodule AshPostgres.MigrationGenerator.Operation.Codec do
   end
 
   defp maybe_to_atom(value) when is_atom(value), do: value
+  # sobelow_skip ["DOS.StringToAtom"]
+  # Snapshot files are developer-committed and reviewed. This matches the
+  # pattern already skipped in migration_generator.ex.
   defp maybe_to_atom(value), do: String.to_atom(value)
 
   defp iso8601_now do

lib/migration_generator/reducer.ex

@@ -75,19 +75,6 @@ defmodule AshPostgres.MigrationGenerator.Reducer do
     end
   end
 
-  @doc """
-  Produce a list of operation structs that, when reduced from an empty state,
-  reconstruct the given existing state.
-
-  Uses the same machinery the live generator uses for brand-new tables — we
-  invoke the migration generator's empty-snapshot diff branch so the ops are
-  byte-identical with what `do_fetch_operations/4` would emit on first-time
-  table creation.
-  """
-  def state_to_initial_delta(state) do
-    AshPostgres.MigrationGenerator.initial_operations_for_state(state)
-  end
-
   @doc "The empty starting-state for a given snapshot (path + multitenancy context)."
   def empty_state(snapshot) do
     %{

lib/mix/tasks/ash_postgres.squash_snapshots.ex

@@ -58,6 +58,7 @@ defmodule Mix.Tasks.AshPostgres.SquashSnapshots do
   @timestamp_regex ~r/^\d{14}\.json$/
   @dev_regex ~r/^\d{14}_dev\.json$/
 
+  @impl Mix.Task
   def run(args) do
     {opts, []} = OptionParser.parse!(args, strict: @switches)
 
@@ -189,7 +190,7 @@ defmodule Mix.Tasks.AshPostgres.SquashSnapshots do
     """
   end
 
-  defp apply_squash({folder, snapshots, last_snapshot, into_snapshot, :full}, _opts) do
+  defp apply_squash({_folder, snapshots, last_snapshot, into_snapshot, :full}, _opts) do
     for snapshot <- snapshots, snapshot != last_snapshot do
       File.rm!(snapshot)
     end
@@ -198,7 +199,6 @@ defmodule Mix.Tasks.AshPostgres.SquashSnapshots do
       File.rename!(last_snapshot, into_snapshot)
     end
 
-    _ = folder
     :ok
   end
 

test/snapshot_delta_test.exs

@@ -174,6 +174,24 @@ defmodule AshPostgres.SnapshotDeltaTest do
       refute Codec.delta?(legacy_json)
     end
 
+    test "rejects malformed JSON with a parseable error message" do
+      assert_raise ArgumentError, ~r/Could not parse delta snapshot JSON/, fn ->
+        Codec.decode_delta("not valid json {[")
+      end
+
+      refute Codec.delta?("not valid json {[")
+    end
+
+    test "rejects JSON that isn't a top-level object" do
+      assert_raise ArgumentError, ~r/Expected delta snapshot to be a JSON object/, fn ->
+        Codec.decode_delta("[1, 2, 3]")
+      end
+
+      assert_raise ArgumentError, ~r/Expected delta snapshot to be a JSON object/, fn ->
+        Codec.decode_delta("\"a string\"")
+      end
+    end
+
     test "pseudo-ops (SetBaseFilter, OptOutDropTable) round-trip" do
       ops = [
         %Operation.SetBaseFilter{