[18.0][IMP][BCKP] base_tier_validation: clearer notify_on_pending chatter body

[bosd]

Backport of OCA/tier-validation#22 to 18.0.

Problem

The chatter message posted when a tier reaches pending (from
_notify_review_available) has two UX problems:

1. Wrong attribution on later tiers. self.env.user.name inside
   _notify_requested_review_body resolves to whichever user
   triggered the promotion. On second-and-later tiers that is the
   approver of the previous tier, not the person who pressed
   Request Validation -- so the message credits the wrong user.
2. No way to tell who the message is for. Chatter system
   notifications do not render the "Notified to" pill the way
   user-typed comments do. A recipient seeing "A review has been
   requested by Administrator" cannot tell whether it was meant for
   them.

Fix

• Source the requester from review.requested_by (set once at
  request_validation time, never reassigned) so attribution is
  consistent across all tiers.
• Reuse the existing tier.review.todo_by field for the assignee
  label -- it already handles every review type (individual /
  res.users field, group, res.groups field) with the same
  abbreviation rules as the Reviews table.
• New wording: "Review pending for Mike, requested by Administrator."

Backwards-compatible: _notify_requested_review_body() still works
with no arguments and returns the previous short body for any
downstream overrider that calls it that way.

Notes on backport scope

The upstream PR also extends test_19a_notify_on_pending_sequence_negative,
which was introduced by its dependency OCA/tier-validation#21 and does
not exist on this branch (18.0's nearest equivalent,
test_19_waiting_tier, deliberately runs with notify_on_pending
off). Only the test_20_no_sequence adaptation -- which asserts the
assignee and the original requester are present in the body -- has been
ported here.

Test plan

• test_20_no_sequence updated to assert both the assignee
  (review_1.todo_by) and the original requester
  (review_1.requested_by) are present in the chatter body.

[OCA-git-bot]

Hi @LoisRForgeFlow,
some modules you are maintaining are being modified, check this out!