support realtime transcription intent

[sarptandoven]

summary

• adds an optional intent: 'transcription' realtime connection option
• includes the intent query parameter in stable and beta realtime URLs for OpenAI and Azure deployment connections
• accepts the same intent option on stable and beta Azure realtime factory helpers

why

• fixes Support realtime transcription #1511
• Azure realtime transcription requires intent=transcription, but the SDK only exposed model / deploymentName and callID connection targets
• callers currently need lower-level URL mutation workarounds to create transcription realtime sessions

validation

• pnpm exec jest tests/realtime.test.ts --runInBand
• pnpm exec tsc --noEmit --pretty false
• git diff --check
• pnpm lint

[sarptandoven]

thanks, that was right. i changed this to model transcription as a separate connection variant instead of adding intent to the model/deployment variant.

updated behavior:

• standard OpenAI transcription now builds ...?intent=transcription, with no model query param
• Azure transcription builds with intent=transcription and no deployment query param
• model/deployment and sideband callID connections keep their existing URL shapes
• model + intent, deploymentName + intent, and callID + intent are rejected instead of silently producing an invalid mixed target

I also updated the realtime tests to cover the intent-only URLs and the invalid mixed-target cases.