improvement: decode an interval as a Duration by default

[matt-beanland]

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Summary

Part of #553 (doesn't close it because operators are outstanding)

A :duration attribute cannot be read on Postgres. The value encodes into the interval column fine, but the RETURNING row loads back as a %Postgrex.Interval{}, which Ash.Type.Duration refuses, then the create fails with an Ash.Error.Unknown and the insert is rolled back, leaving nothing stored.

postgrex can already decode an interval as a Duration, but interval_decode_type is only settable at Postgrex.Types.define/3, never in repo configuration, and ash_postgres shipped no types module. This adds AshPostgres.PostgrexTypes and applies it in AshPostgres.Repo.init/2 with Keyword.put_new, so a repo defining its own module for other extensions keeps it.

The migration generator needs no change: a :duration attribute already generates add(:field, :duration), which Ecto renders as interval. In Postgres INTERVAL and INTERVAL(6) are identical with microsecond precision.

This changes interval decoding for every repo, not only :duration attributes: raw queries and any custom interval-mapped type will receive a Duration rather than a %Postgrex.Interval{}. There is no Ash :duration data to migrate, since loading one currently fails.

Best paired with ash#2851 — without it, a units-constrained duration is readable but not re-writable once persisted.

Tested with Postgres 19.

[zachdaniel]

This could theoretically break any custom code that a user has. In other cases, we require the user to do this in their own application so that they have a single authoritative source of truth. I think we will have to do that here as well otherwise risk anyone who has custom interval handling code that is also using AshPostgres.

[matt-beanland]

Yes, agreed. The default is gone, and a repo's types: is never touched.

Now:

• The error message teaches - A %Postgrex.Interval{} reaching Ash.Type.Duration now appends
  the Postgrex.Types.define/3 call and the repo config to the message, instead of a cast
  error naming neither.

• The tests show how - AshPostgres.TestRepo configures its own types module exactly as
  the docs instruct, so the documented path is the one CI exercises rather than only
  describes.

• The solution is pinned - Tests assert an interval loads as a Duration, that
  months and days survive separately, and that a repo configuring its own types: keeps it.

(GitHub has followed this comment onto the test-support module — the file it was on is
deleted.)