stt: scope websocket receive loops

[yisding]

Problem

WebSocket STT receive loops still use a raw task spawn. They cannot share the earlier stt-runtime cancel phase because the receive loop must remain live until provider finalization sends its control frame and closes or releases the socket.

Changes

• own receive loops under the attached STT provider RuntimeScope
• add a distinct stt-receive finish cohort ordered after stt-finalize
• preserve the existing provider-local close timeout and cancellation fallback
• document the new phase in the complete Session.stop mapping
• remove the final raw task spawn from WebSocketSTTBase's connect path

Validation

• just check — 8522 passed, 308 skipped
• focused WebSocket STT matrix — 244 passed, 3 deselected
• focused Ruff and mypy checks
• source and teardown-budget ratchets

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[chatgpt-codex-connector]

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