fix(trader): add backend safety checks for partial_close (#713)

* fix(trader): add backend safety checks for partial_close

After PR #415 added partial_close functionality, production users reported two critical issues:

1. **Exchange minimum value error**: "Order must have minimum value of $10" when remaining position value falls below exchange threshold
2. **Unprotected positions after partial close**: Exchanges auto-cancel TP/SL orders when position size changes, leaving remaining position exposed to liquidation risk

This PR adds **backend safety checks** as a safety net layer that complements the prompt-based rules from PR #712.

**Protection**: Before executing partial_close, verify remaining position value > $10

```go
const MIN_POSITION_VALUE = 10.0 // Exchange底线
remainingValue := remainingQuantity * markPrice

if remainingValue > 0 && remainingValue <= MIN_POSITION_VALUE {
    // 🔄 Auto-correct to full close
    decision.Action = "close_long" // or "close_short"
    return at.executeCloseLongWithRecord(decision, actionRecord)
}
```

**Behavior**:
- Position $20 → partial_close 50% → remaining $10 ≤ $10 → Auto full close ✅
- Position $30 → partial_close 50% → remaining $15 > $10 → Allow partial close ✅

**Protection**: Restore TP/SL orders for remaining position if AI provides new_stop_loss/new_take_profit

```go
// Exchanges auto-cancel TP/SL when position size changes
if decision.NewStopLoss > 0 {
    at.trader.SetStopLoss(symbol, side, remainingQuantity, decision.NewStopLoss)
}
if decision.NewTakeProfit > 0 {
    at.trader.SetTakeProfit(symbol, side, remainingQuantity, decision.NewTakeProfit)
}

// Warning if AI didn't provide new TP/SL
if decision.NewStopLoss <= 0 && decision.NewTakeProfit <= 0 {
    log.Printf("⚠️⚠️⚠️ Warning: Remaining position has NO TP/SL protection")
}
```

**Improvement**: Show position quantity and value to help AI make better decisions

```
Before: | 入场价100.00 当前价105.00 | 盈亏+5.00% | ...
After:  | 入场价100.00 当前价105.00 | 数量0.5000 | 仓位价值52.50 USDT | 盈亏+5.00% | ...
```

**Benefits**:
- AI can now calculate: remaining_value = current_value × (1 - close_percentage/100)
- AI can proactively avoid decisions that would violate $10 threshold

- Added MIN_POSITION_VALUE check before execution
- Auto-correct to full close if remaining value ≤ $10
- Restore TP/SL for remaining position
- Warning logs when AI doesn't provide new TP/SL

- Import "math" package
- Calculate and display position value
- Add quantity and position value to prompt

- Complements PR #712 (Prompt v6.0.0 safety rules)
- Addresses #301 (Backend layer)
- Based on PR #415 (Core functionality)

| Layer | Location | Purpose |
|-------|----------|---------|
| **Layer 1: AI Prompt** | PR #712 | Prevent bad decisions before they happen |
| **Layer 2: Backend** | This PR | Auto-correct and safety net |

**Together they provide**:
- ✅ AI makes better decisions (sees position value, knows rules)
- ✅ Backend catches edge cases (auto-corrects violations)
- ✅ User-friendly warnings (explains what happened)

- [x] Compiles successfully (`go build ./...`)
- [x] MIN_POSITION_VALUE logic correct
- [x] TP/SL restoration logic correct
- [x] Position value display format validated
- [x] Auto-correction flow tested

This PR can be merged **independently** of PR #712, or together.

Suggested merge order:
1. PR #712 (Prompt v6.0.0) - AI layer improvements
2. This PR (Backend safety) - Safety net layer

Or merge together for complete two-layer protection.

---

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

* fix: add error handling for markPrice type assertion

- Check type assertion success before using markPrice
- Return error if markPrice is invalid or <= 0
- Addresses code review feedback from @xqliu in PR #713

* test(trader): add comprehensive unit tests for partial_close safety checks

- Test minimum position value check (< 10 USDT triggers full close)
- Test boundary condition (exactly 10 USDT also triggers full close)
- Test stop-loss/take-profit recovery after partial close
- Test edge cases (invalid close percentages)
- Test integration scenarios with mock trader

All 14 test cases passed, covering:
1. MinPositionCheck (5 cases): normal, small remainder, boundary, edge cases
2. StopLossTakeProfitRecovery (4 cases): both/SL only/TP only/none
3. EdgeCases (4 cases): zero/over 100/negative/normal percentages
4. Integration (2 cases): LONG with SL/TP, SHORT with auto full close

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

* style: apply go fmt after rebase

Only formatting changes:
- api/server.go: fix indentation
- manager/trader_manager.go: add blank line
- trader/partial_close_test.go: align struct fields

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

* fix(test): rename MockTrader to MockPartialCloseTrader to avoid conflict

Problem:
- trader/partial_close_test.go defined MockTrader
- trader/auto_trader_test.go already has MockTrader
- Methods CloseLong, CloseShort, SetStopLoss, SetTakeProfit were declared twice
- Compilation failed with 'already declared' errors

Solution:
- Rename MockTrader to MockPartialCloseTrader in partial_close_test.go
- This avoids naming conflict while keeping test logic independent

Test Results:
- All partial close tests pass
- All trader tests pass

Related: PR #713

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

---------

Co-authored-by: ZhouYongyou <128128010+zhouyongyou@users.noreply.github.com>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: the-dev-z <the-dev-z@users.noreply.github.com>

decision/engine.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"log"
+	"math"
 	"nofx/market"
 	"nofx/mcp"
 	"nofx/pool"
@@ -397,9 +398,12 @@ func buildUserPrompt(ctx *Context) string {
 				}
 			}
 
-			sb.WriteString(fmt.Sprintf("%d. %s %s | 入场价%.4f 当前价%.4f | 盈亏%+.2f%% | 盈亏金额%+.2f USDT | 最高收益率%.2f%% | 杠杆%dx | 保证金%.0f | 强平价%.4f%s\n\n",
+			// 计算仓位价值（用于 partial_close 检查）
+			positionValue := math.Abs(pos.Quantity) * pos.MarkPrice
+
+			sb.WriteString(fmt.Sprintf("%d. %s %s | 入场价%.4f 当前价%.4f | 数量%.4f | 仓位价值%.2f USDT | 盈亏%+.2f%% | 盈亏金额%+.2f USDT | 最高收益率%.2f%% | 杠杆%dx | 保证金%.0f | 强平价%.4f%s\n\n",
 				i+1, pos.Symbol, strings.ToUpper(pos.Side),
-				pos.EntryPrice, pos.MarkPrice, pos.UnrealizedPnLPct, pos.UnrealizedPnL, pos.PeakPnLPct,
+				pos.EntryPrice, pos.MarkPrice, pos.Quantity, positionValue, pos.UnrealizedPnLPct, pos.UnrealizedPnL, pos.PeakPnLPct,
 				pos.Leverage, pos.MarginUsed, pos.LiquidationPrice, holdingDuration))
 
 			// 使用FormatMarketData输出完整市场数据