fix(gateway): include caching in GatewayProviderOptions

[MukundaKatta]

Summary

• add the missing caching field to the gateway provider options schema
• cover the schema change with a gateway-language-model regression test
• align the exported TypeScript type with the documented automatic caching configuration

Closes #14595

Verification

• pnpm vitest run packages/gateway/src/gateway-language-model.test.ts -t "allow caching in provider options" (not runnable here because vitest is not installed in the current workspace dependencies and this environment uses unsupported Node v25)

[MukundaKatta]

#14664 addresses the same issue with a more complete diff. Closing this earlier version to keep review signal clean.