Define Realtime routing for transcription terminals without item metadata

[andimarafioti]

Context

PR #476 fixes #474 by translating cumulative internal STT hypotheses into incremental Realtime transcription deltas and routing normal turn events by item.

During implementation, it also introduced a broader policy for authoritative TranscriptionCompletedEvent values that cannot be matched to a speech lifecycle item—for example, terminals emitted without turn_id / turn_revision, terminals emitted without a preceding SpeechStartedEvent, or metadata-less terminals arriving while multiple input items are unresolved.

That fallback policy is not required to fix #474 and should be designed separately.

Scope

• Define how a transcription terminal without a matching speech lifecycle item is represented in the Realtime conversation.
• Decide whether and when the server may synthesize a standalone input item.
• Define routing for metadata-less terminals when zero, one, or multiple input items are unresolved.
• Preserve the Realtime contract that the item_id announced by input_audio_buffer.speech_started remains the identity used by matching transcription events.
• Document compatibility behavior for protocol-neutral pipelines that omit turn metadata.
• Add focused normal, missing-lifecycle, and ambiguous-overlap regressions.

Acceptance criteria

• A terminal that belongs to an announced speech item does not switch to a new item ID or lose its recorded duration.
• An ambiguous terminal never silently corrupts another item's transcript.
• Any synthesized item has an explicit, documented protocol lifecycle.
• The fallback behavior is deterministic and tested without changing incremental delta semantics.

Non-goals

• Converting cumulative partial hypotheses to incremental deltas; tracked by Align Realtime input transcription delta semantics #474.
• Bounding retained input-transcription state; tracked separately.
• Changing STT backend behavior.