Malformed `cost.tiers` still crashes the turn (TypeError, not DecimalError)

[macurandb]

Description

#43248 guarded the cost operands, but the tier selection right above them is still unguarded:

input.model.cost?.tiers
  ?.filter((item) => item.tier.type === "context" && contextTokens > item.tier.size)

cost() in provider/provider.ts copies tier: item.tier straight from the models.dev / plugin payload with no guard of its own — unlike input/output/cache. So a malformed tiers still throws from getUsage and takes the turn with it. Same symptom as #43098 (fatal under opencode acp), different error:

cost.tiers	error on dev
[{ input, output, cache }] (no tier key)	TypeError: undefined is not an object (evaluating 'item.tier.type')
[null]	TypeError: null is not an object (evaluating 'item.tier')
{} (not an array)	TypeError: input.model.cost?.tiers?.filter is not a function

Note a non-numeric tier.size must not be coerced to 0 when fixing this — that would make the tier match every context instead of none.

OpenCode version

Reproduces on current dev (9b0dd36 — i.e. with #43248 already in).

Steps to reproduce

Call getUsage with a model whose cost.tiers is any of the three shapes above. Three failing unit tests in the PR that follows.

Operating System

macOS

Terminal

VS Code integrated terminal