docs(api): clarify burst and daily rate limits

[JacobisEpic]

Fixes #6595

Summary

Correct the rate-limit documentation to match the current gateway contract after #4635 replaced the former 10× safety ceiling with hard enforcement at each plan's sold daily allowance.

Documentation changed

• Separate per-minute burst limits from daily plan limits, including their 429 and reset behavior.
• State that usage-based overage billing is not active and link epic: usage-based overage billing on Dodo (Phase 2 of #3199) #4560 as its roadmap.
• Document the existing REST SDK transport-error behavior and the per_minute / daily limit_type values.
• Align the Chinese rate-limits page with the current contract.

Validation

• npm run --silent docs:check
• npm run --silent lint:public-docs
• npm run --silent lint:mintlify-slugs
• npm run --silent lint:unicode
• tsx --test tests/api-key-rate-limit.test.mts
• vitest run server/__tests__/gateway-api-rate-limit.test.ts
• git diff --check

Docs-only change.

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