fix: recognize extra usage as long-context fallback

[rich-jojo]

Summary

• treat You're out of extra usage responses as long-context fallback signals inside isLongContextError()
• let the existing 1M beta exclusion path retry at 200k context instead of failing Opus/Max requests outright
• add regression coverage for the extra-usage response in src/betas.test.ts

Related issue

Primary issue: #159
Closes #159.
Helps #188.

Testing

• corepack pnpm run format
• corepack pnpm run lint
• corepack pnpm run build
• node --test --experimental-strip-types src/betas.test.ts
• corepack pnpm test (still fails on pre-existing ESM temp-module loading errors in src/credentials.test.ts and src/index.test.ts under this local Node 24 environment)

Checklist

• PR title follows Conventional Commits (feat:, fix:, docs:, chore:, etc.)
• make all passes locally (runs lint, build, and test)
• Tests added or updated where applicable
• README or docs updated where applicable