fix(价格展示): 修复模板渲染时极小价格被折叠为 0.00 的问题

Motivation:
此前的修复仅覆盖直接拼接文案的场景，跟单等基于模板渲染的推送场景中，%.2f 格式化仍会把 PEPE 等 Meme 币的极小价格（如 0.00000059）折叠为 0.00，导致交易通知丢失真实价格、误导用户。

Changes:

* 抽取共享的价格显示逻辑到统一显示层，两个订阅者改为复用，移除重复实现
* 新增 Decimal 类型，渲染时以副本方式包装模板数据，使模板内 printf 风格格式化保留极小价格精度且不出现科学计数法，同时不修改调用方原始数据
* 渲染引擎空值判断改用反射实现，覆盖全部整型、无符号整型与浮点类型
* 补充转换到渲染的端到端回归测试，覆盖极小价格精度、科学计数法与数据不可变性

internal/subscriber/tradesignal/convert_test.go

@@ -6,6 +6,7 @@ import (
 	"testing"
 
 	"aiaa-notification-service/internal/config"
+	"aiaa-notification-service/internal/engine"
 )
 
 func TestConvertOpen(t *testing.T) {
@@ -97,3 +98,43 @@ func TestConvertRawMessageWithoutAction(t *testing.T) {
 		t.Fatalf("data=%v", data)
 	}
 }
+
+func TestConvertThenRenderCopyTradeTinyPEPE(t *testing.T) {
+	lev := 100
+	c := NewConverter(map[string]config.StrategyOverride{
+		"BLONG": {QuantityMultipliers: config.QuantityMultipliers{Open: 100}, Leverage: &lev},
+	})
+	_, data, err := c.Convert([]byte(`{
+		"signalId":"s1","strategyCode":"BLONG","symbol":"PEPEUSDT",
+		"side":"SHORT","action":"OPEN","quantity":10000000,"price":0.00000059,
+		"leverage":10,"eventTime":"2026-08-22T02:04:29Z"
+	}`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if p, ok := data["price"].(float64); !ok || p != 0.00000059 {
+		t.Fatalf("convert price=%v (%T), want float64 0.00000059", data["price"], data["price"])
+	}
+	data["pushedAt"] = "2026.08.22 10:04:29"
+
+	const tmpl = "{{case .side \"LONG\" \"多单\" \"SHORT\" \"空单\"}}{{case .action \"OPEN\" \"开仓\" \"ADD\" \"加仓\" \"CLOSE\" \"平仓\" \"REDUCE\" \"减仓\"}}\n交易品种: {{case .symbol \"ETHUSDT\" \"ETH\" \"BTCUSDT\" \"BTC\" \"SOLUSDT\" \"SOL\" \"BNBUSDT\" \"BNB\" .symbol}}\n{{case .action \"OPEN\" \"开仓价格\" \"ADD\" \"加仓价格\" \"CLOSE\" \"平仓价格\" \"REDUCE\" \"减仓价格\"}}: {{printf \"%.2f\" .price}}\n{{case .action \"OPEN\" \"开仓数量\" \"ADD\" \"加仓数量\" \"CLOSE\" \"平仓数量\" \"REDUCE\" \"减仓数量\"}}: {{printf \"%.2f\" .quantity}}\n平均单价: {{printf \"%.2f\" .avgPrice}}\n{{if or (eq .action \"OPEN\") (eq .action \"ADD\")}}{{if .leverage}}杠杆: {{.leverage}}x\n{{end}}{{end}}策略: {{case .strategyCode \"BLONG\" \"B龙策略\" .strategyCode}}\n推送时间: {{.pushedAt}}"
+	out, err := engine.NewRenderer().Render(tmpl, data)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, want := range []string{
+		"空单开仓",
+		"交易品种: PEPEUSDT",
+		"开仓价格: 0.00000059",
+		"开仓数量: 1000000000.00",
+		"平均单价: 0.00000059",
+		"杠杆: 100x",
+	} {
+		if !strings.Contains(out, want) {
+			t.Errorf("missing %q in\n%s", want, out)
+		}
+	}
+	if strings.Contains(out, "开仓价格: 0.00\n") || strings.Contains(out, "平均单价: 0.00\n") {
+		t.Errorf("tiny price collapsed to 0.00:\n%s", out)
+	}
+}