stt: scope interruptible audio sends

[yisding]

Problem

Interruptible STT audio writes are still spawned directly and tracked only by a provider-local task handle, so Session teardown cannot see that work in its runtime tree.

Changes

• add a typed RuntimeScopeBindable provider capability
• attach built-in STT provider runtime work beneath the Session root
• place interruptible audio writes in the mapped stt-runtime cancellation cohort
• preserve the existing child-vs-caller cancellation discriminator and retained-send retry behavior
• close idle local scopes for standalone provider use
• remove one raw task spawn and one gather(return_exceptions=True) site from the source ratchet

WebSocket receive-loop migration remains separate because that loop must stay live through provider finalization; folding it into this cohort without its finalizer ordering would lose final transcripts.

Validation

• just check — 8521 passed, 308 skipped
• focused STT/Session lifecycle suite — 95 passed
• focused Ruff and mypy checks
• source and teardown-budget ratchets

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