[19.0][IMP] base_tier_validation: name the assignee in the "to validate" banner

[bosd]

What

The yellow banner shown above a tier-validated document currently reads

This Record needs to be validated.

…which is generic in two ways: it does not say which model the record belongs to, and -- more importantly -- it does not say who is expected to act. Users in our rollout consistently asked "who's holding this up?" even though that information already exists on tier.review.todo_by.

This PR makes the banner read, for example:

Pending validation by John
Pending validation by Group Accountants
Pending validation by Mike, Jane (and 2 more)

…sourced from the same todo_by the Reviews table already uses, so individual users, groups, res.users / res.groups field reviewers and Python-expression reviewers all render uniformly.

When no review has yet reached pending (defensive edge case -- e.g. waiting state, or downstream code that calls the method on a fresh record), the banner falls back to the existing model-name phrasing:

This Sale Order needs to be validated.

Plumbing

• _get_to_validate_message now picks the lowest-sequence pending review and embeds its todo_by into the body.
• _compute_to_validate_message now depends on review_ids.status and review_ids.todo_by in addition to validation_status, so the banner refreshes when a tier is promoted or when the assignee changes.
• The tier_validation_label template now actually renders the to_validate_message Html field (which has existed on the abstract model since v14 but was never wired into the template; the template carried its own hardcoded text).

Tests

• test_19c_to_validate_message_names_assignee -- after a tier reaches pending, the banner contains the assignee and does not contain the generic phrasing.
• test_19d_to_validate_message_falls_back_when_no_pending -- with no pending review on the record, the banner falls back to the model-name phrasing.

Notes

• Independent of [19.0][FIX] base_tier_validation: auto-promote first review to pending #21, [19.0][IMP] base_tier_validation: clearer notify_on_pending chatter body #22, and [19.0]IMP] base_tier_validation: UI clean-up #23 -- no overlap. Touches model + template + tests only.
• _get_to_validate_message_name is unchanged and still hooks any downstream override (custom phrasing per model). The new code only changes what's rendered when todo_by is available.
• next_review (the "Next: " sub-line) is left as-is. It's complementary information -- the new banner says who, next_review says which tier. Removing it is out of scope.

CC @LoisRForgeFlow

[OCA-git-bot]

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

[bosd]

Now one can see exactly who it is waitiing on, when doing a sequenced validation:

Before:

After: