docs(provider): MiniMax dual-endpoint auto-switch + regression (#141)

[quangdang46]

What

Documents and pins down our forks dual-endpoint behavior for MiniMax,
which already addresses the underlying user complaint in
issue #141 (#141)
without the regression cost of upstreams approach.

Background

MiniMax operates two separate platforms:

Platform	Endpoint	API key style
International	https://api.minimax.io/v1	normal
China Token Plan	https://api.minimaxi.com/v1	sk-cp-...

Upstream PR 1jehuang#188 fixes the
reported 401 authorized_error for China Token Plan users by flipping
the default MINIMAX_PROFILE.api_base to the China URL.

This forks resolver
(apply_profile_key_based_endpoint_overrides in
crates/jcode-provider-metadata /
src/provider_catalog.rs) already auto-routes any sk-cp-* API key
to MINIMAX_CHINA_API_BASE without changing the default for
international users. Mechanically porting upstreams flip would
regress every non-sk-cp-* MiniMax user. So we deviate.

Changes

• OAUTH.md: rewrite the MiniMax section to explicitly document
  both endpoints, the sk-cp-* auto-switch, and the matching docs
  URL for each platform. This is the user-visible piece that closes
  the gap behind the original 401 reports.
• src/provider_catalog_tests.rs: add
  minimax_token_plan_keys_route_to_china_even_when_openai_api_key_env_is_international
  — pins down the exact failure scenario from the issue: an unrelated
  international OPENAI_API_KEY set in env must NOT pull the
  resolution back to .io when a real sk-cp-* key is the one being
  used.
• MINIMAX_PROFILE defaults are intentionally unchanged.

Tests

$ cargo test -p jcode --lib provider_catalog_tests::minimax_
test result: ok. 2 passed; 0 failed; 0 ignored

The pre-existing
minimax_token_plan_keys_resolve_to_china_endpoint_without_changing_international_default
plus the new regression test together lock the contract.

Acceptance criteria

• China Token Plan keys (sk-cp-...) hit api.minimaxi.com —
  pinned by both tests.
• International keys still hit api.minimax.io — pinned by the
  pre-existing test.
• Unrelated OPENAI_API_KEY in env cannot poison the resolution
  — pinned by the new test.
• User-facing docs (OAUTH.md) explain both endpoints and the
  auto-switch.

Notes / deviations from upstream

• We do not flip MINIMAX_PROFILE.api_base from .io to .com.
  Upstream PR feat(tui/clipboard): multi-backend copy with OSC 52 fallback (#65) #188 makes that change unconditionally, which would
  break international users whose keys do not start with sk-cp-. Our
  fork already routes by key prefix, which is strictly more correct.
• We do not rename api_key_env from OPENAI_API_KEY to
  MINIMAX_API_KEY here; that is a separate breaking change and
  should be its own PR if desired.

Refs upstream PR 1jehuang#188 by @richardiitse.