Add beta realtime cached token detail types

[MukundaKatta]

Summary

• add cached_tokens_details to OpenAI.Beta.Realtime.RealtimeResponseUsage.InputTokenDetails
• include nested cached token detail fields for text and audio tokens
• add a compile-time/runtime type assertion covering the beta realtime usage shape

Fixes #1600.

Tests

• yarn test tests/betaRealtimeTypes.test.ts

Notes

• yarn build could not complete locally on Node v26.0.0 because tsc-multi/yargs fails with ReferenceError: require is not defined in ES module scope before type-checking.
• ./node_modules/.bin/tsc --noEmit -p tsconfig.json is blocked by optional example dependencies missing locally (@azure/identity, express, next).