feat(ai): switch xiaomi default to api billing, add per-region token plan providers (#4112)

Built-in `xiaomi` provider now targets the API billing endpoint (https://api.xiaomimimo.com/anthropic) — a single stable URL for keys issued at platform.xiaomimimo.com. The Token Plan endpoints are exposed as three sibling providers, each with its own env var:

- xiaomi-token-plan-cn: XIAOMI_TOKEN_PLAN_CN_API_KEY
- xiaomi-token-plan-ams: XIAOMI_TOKEN_PLAN_AMS_API_KEY
- xiaomi-token-plan-sgp: XIAOMI_TOKEN_PLAN_SGP_API_KEY

BREAKING CHANGE: users who previously set XIAOMI_API_KEY against the Token Plan AMS endpoint must move to xiaomi-token-plan-ams and set XIAOMI_TOKEN_PLAN_AMS_API_KEY. This also resolves the 401 reported by on #4005, where a platform.xiaomimimo.com key fails against the Token Plan endpoint.

closes #4082

packages/ai/CHANGELOG.md

@@ -2,6 +2,14 @@
 
 ## [Unreleased]
 
+### Breaking Changes
+
+- Switched the built-in `xiaomi` provider endpoint from Token Plan AMS (`https://token-plan-ams.xiaomimimo.com/anthropic`) to API billing (`https://api.xiaomimimo.com/anthropic`). `XIAOMI_API_KEY` now refers to the API billing key from [platform.xiaomimimo.com](https://platform.xiaomimimo.com). Users still on Token Plan must move to the appropriate `xiaomi-token-plan-*` provider and set the corresponding env var.
+
+### Added
+
+- Added Xiaomi MiMo Token Plan regional providers with per-region env vars: `xiaomi-token-plan-cn` (`XIAOMI_TOKEN_PLAN_CN_API_KEY`), `xiaomi-token-plan-ams` (`XIAOMI_TOKEN_PLAN_AMS_API_KEY`), and `xiaomi-token-plan-sgp` (`XIAOMI_TOKEN_PLAN_SGP_API_KEY`).
+
 ## [0.72.1] - 2026-05-02
 
 ## [0.72.0] - 2026-05-01