fix: Add _open_positions guard in MarketMakingStrategy.close_position()

[keitaj]

Summary

• Override close_position() in MarketMakingStrategy to check _closer._open_positions before sending reduce_only order
• Closes the remaining race condition gap: PR fix: Add final _open_positions guard before all reduce_only orders #116 guarded the PositionCloser paths but not the BaseStrategy close path (used by close_immediately=True and QUIET hours)
• _open_positions is updated by FillFeed on the WS thread with zero cache delay, more reliable than the 2-second TTL position cache

Changes

• strategies/market_making_strategy.py: Add close_position() override with _open_positions guard
• tests/test_close_position_guard.py: 3 new tests (skip when not tracked, proceed when tracked, skip for untracked coin)
• tests/test_reduce_only_force_close.py: Add _closer mock with _open_positions to existing test helper

Test plan

• Unit tests added (3 new)
• flake8 passes
• pytest passes (712 tests)
• No regressions

🤖 Generated with Claude Code

[keitaj]

Review: close_position() _open_positions guard

Clean 11-line override that closes the last remaining reduce_only race condition gap. PR #116 covered PositionCloser paths; this covers the BaseStrategy close path used by close_immediately and QUIET hours.

The guard is consistent with the pattern established in PR #116: check _open_positions (updated instantly by FillFeed) rather than relying on the 2-second TTL cache.

Verdict: LGTM

Minimal change, correct logic, good test coverage. Ship it.