feat(frontend): continue a Tavus conversation across competencies

[micio86dev]

What

FE-04 of tavus-single-session-interview: the client side of the continuation. Two commits.

1. chore: sync openapi.json and the generated types with the merged api contract (live_conversation_id request field, continuation, conversation_id; no conversation_ttl_seconds yet, that is API-05/FE-06). Generated with bun run codegen, not hand-typed.
2. feat: useInterviewSession continues a live Tavus conversation across competencies. isValidStartResponse accepts continuation/conversation_id and rejects a malformed continuation or one combined with a handle; createProvider runs once across three competencies (players.length <= 1); /start carries live_conversation_id only from the 'boundary' target while a handle is joined, never after reload, pause, tab-hidden, re-offer, retry() or embed re-entry; assertBoundary() is idempotent (racing inputs mint one ticket and one /end, a losing 409 unmutes and is a no-op); the mic is muted before /end and unmuted only at the steering ack or steering_failed (resend once for timeout/send_failed, otherwise the new competency ends as timeout); the 'boundary' target neither enters connecting nor publishes a handle; the Tavus continue directive leaves confirmDevices() only when continuation came back. Pause is disabled while a boundary is in flight; the 300 s timer goes through the same assertBoundary().

Decisions

• No new flag: a handle is retargeted in place only when it is Tavus, the server named its conversation and the provider can steer context; otherwise today's path runs unchanged. HeyGen specs are byte-identical to develop.
• A malformed continuation goes to the retryable error screen; a continuation for another conversation, or on a caller without a handle, is refused.
• If the server answers a boundary /start with a fresh handle instead of a continuation, the old provider is stopped and the new handle published.

Verification

RED first (23 of 31 new tests failed; the 8 that passed are negative guards that already held); 31/31 green. Mutations (send the id after pause, skip the mute, drop the in-flight guard) each fail tests and were reverted. Full Vitest 117 files / 2977 tests, typecheck, lint (0 errors) pass. gga passed. Not verified: coverage of this file after the final edits (CI gates it), real Tavus/Daily, Playwright (FE-07).