fix(turn): clear active slot on cancel to avoid phantom -32003

[icn5381]

Summary

turn.cancel relied on the sink's on_turn_end callback (fired from _drop at turn exit) to clear the _active_turns slot, and awaited handle.result() assuming the sink had already dropped it. A cancelled turn can resolve before that callback runs, so the slot was sometimes left populated and the next turn.send hit a phantom turn_in_progress (-32003).

Call clear_active from turn_cancel itself after the drain. pop is idempotent, so the sink's later clear is a harmless no-op.

Type

• Fix

Verification

• Relevant tests pass locally: uv run pytest tests/test_turn_cancel_clears_active.py -v -> 2 passed
• Relevant lint / type checks pass locally: uv run ruff format (2 files unchanged), uv run ruff check (all passed)
• User-facing docs or screenshots updated: N/A (internal lifecycle fix)

Risk

• Security impact considered: only clears an in-memory turn slot the caller already owns; no new input, no shell
• Backward compatibility considered: clear_active pops with a default and is already the sink's turn-end path, so the added call is additive and idempotent
• Rollback path is clear: revert restores the sink-only clear

Related Issues

Fixes #115