Retain existing leveraged ETF shares for small accounts

[Pigbibi]

Summary

• keep an existing whole share for TQQQ/SOXL when the positive target is below one share price
• preserve existing cash-substitution behavior for other unbuyable sleeves
• add regression coverage for the SG-style sell-all-then-cannot-buy-back case

Tests

• python -m py_compile application/execution_service.py tests/test_rebalance_service.py
• git diff --check
• timeout 120 python -m pytest tests/test_rebalance_service.py -k 'existing_tqqq_retention_below_one_share or small_account_whole_share_layer_sells_unbuyable_soxx_sleeve or tqqq_delevered_qqqm_target_is_executable or tqqq_delevered_qqq_target_below_one_share' -q

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Avoid retaining an extra share when quotes tick up

When the order quote is even slightly above the portfolio snapshot's per-share valuation, setting the retained target to exactly price makes _sell_order_quantity compute (current_value - price) / price and then floor it below held - 1; for example, 7 TQQQ shares valued at $541.31 with a fresh quote of $77.34 sells only 5 shares, leaving 2 instead of the intended minimum whole share. Derive the retained target/order quantity from the held quantity, or otherwise account for quote/snapshot drift before flooring.

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