bug: Demonstrate AshPostgres.Timestamptz operator overloading causing precision loss in keyset pagination

[sevenseacat]

Discovered while investigating sevenseacat/cinder#176 and tracing it back to a problem upstream.

The failing test demonstrates the problem - sorting by the timestamp causes issues when navigating between pages, as precision is lost in the timestamp comparison.

The root cause appears to be ash_postgres/lib/types/timestamptz.ex:44-79 - operator_overloads/0 overloads only <, <=, >, >=, and the match check via Ash.Expr.determine_types uses matches_type?, which is permissive enough that any %DateTime{} value matches the Timestamptz slot. So users with plain UtcDatetimeUsec columns get their ordering comparisons silently force-cast to seconds-precision Timestamptz, while their equality comparisons keep microsecond precision.

Demo:

iex(1)> resource = CinderSort.Locations.Location
CinderSort.Locations.Location

iex(2)> attr = Ash.Resource.Info.field(resource, :inserted_at)
%Ash.Resource.Attribute{
  name: :inserted_at,
  type: Ash.Type.UtcDatetimeUsec,
  ...
  }
}

iex(3)> Ash.Type.storage_type(attr.type, attr.constraints)
:utc_datetime_usec

iex(4)> ref = %Ash.Query.Ref{attribute: attr, relationship_path: [], resource: resource}
inserted_at

iex(5)> value = ~U[2026-05-25 05:34:24.130812Z]
~U[2026-05-25 05:34:24.130812Z]

iex(6)> Ash.Expr.determine_types(Ash.Query.Operator.Eq, [ref, value])
{[
   {Ash.Type.UtcDatetimeUsec,
    [precision: :microsecond, cast_dates_as: :start_of_day, timezone: :utc]},
   {Ash.Type.UtcDatetimeUsec,
    [precision: :microsecond, cast_dates_as: :start_of_day, timezone: :utc]}
 ], {Ash.Type.Boolean, []}}

iex(7)> Ash.Expr.determine_types(Ash.Query.Operator.LessThan, [ref, value])
{[{AshPostgres.Timestamptz, []}, {AshPostgres.Timestamptz, []}],
 {Ash.Type.Boolean, []}}

iex(8)> Ash.Expr.determine_types(Ash.Query.Operator.GreaterThan, [ref, value])
{[{AshPostgres.Timestamptz, []}, {AshPostgres.Timestamptz, []}],
 {Ash.Type.Boolean, []}}

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