bug(provider): Bedrock au. prefix skips Claude Opus in ap-southeast-2/4

Summary

In ap-southeast-2 / ap-southeast-4, the Bedrock provider only applies the au. cross-region inference prefix to Sonnet 4.5 and Haiku. Claude Opus (and Sonnet 4.6) fall through to the generic apac. prefix, producing a model identifier that does not exist in the Australia geography.

Evidence

packages/opencode/src/provider/provider.ts (AU branch of getModel):

const isAustraliaRegion = ["ap-southeast-2", "ap-southeast-4"].includes(region)
...
if (
  isAustraliaRegion &&
  ["anthropic.claude-sonnet-4-5", "anthropic.claude-haiku"].some((m) => modelID.includes(m))
) {
  regionPrefix = "au"
  modelID = `${regionPrefix}.${modelID}`
} else {
  // Other APAC regions use apac. prefix
  ...
  regionPrefix = "apac"
}

An Opus model ID does not match the allowlist, so it is prefixed apac. instead of au., and Bedrock rejects it in the AU regions.

AWS documents Claude Opus 4.6, Sonnet 4.6, and Haiku 4.5 as available via cross-region inference in the APAC/Australia geography: https://aws.amazon.com/blogs/machine-learning/global-cross-region-inference-for-latest-anthropic-claude-opus-sonnet-and-haiku-models-on-amazon-bedrock-in-thailand-malaysia-singapore-indonesia-and-taiwan/

Impact

Selecting an Opus (or Sonnet 4.6) Bedrock model in ap-southeast-2/ap-southeast-4 fails with an invalid model identifier error. Setting the model ID explicitly with an au. prefix works because of the existing crossRegionPrefixes passthrough, but automatic prefixing does not.

Prior context

• #15182 (AU Opus 4.6 / Sonnet 4.6 request) — closed as duplicate, no fix.
• #12824 (APAC invalid model identifier) — closed stale; documents the same root cause.
• #3050 (merged) added au. for Sonnet 4.5 / Haiku only.

Proposed fix

Broaden the AU match to the claude family for the Australia regions (mirroring the eu/apac branches) rather than maintaining a per-version allowlist that goes stale. PR to follow.