feat(billing): reasoning 价接入 DSL 与规则编辑器

上一提交把链路打通后，reasoning_price 只能靠手改 prices 生效 —— 而 prices 是
编译产物，不是人能编辑的东西。这条补上让它真正可用：

- BillingToken 加 reasoning 字段（USD/百万，与 prompt/completion 同单位）
- compile 只在规则显式声明时写 reasoning_price，缺省时字段不出现
- 规则编辑器的模型行加「思考 $/M」输入框，含新增行的模板

缺省时「不出现」是刻意的：写 0 会被 billing 插件读成「思考免费」，
每个来自该模型的请求都会静默少算。判据专门钉住这一点，变异成「缺省写 0」
即变红。

输入框 placeholder 写明「留空=按输出价计（OpenAI/DeepSeek/Anthropic 都这样）」，
因为一个没有解释的空输入框会被当成「必填但我不知道填什么」。

判据还要求 m-r 出现两次 —— 只改已有行的模板而不改「+ 模型」新建的行，
会让新加的模型在下一次保存时静默丢掉这个价。

生产验证：PUT 一个 reasoning: "1.80" 的规则，编译产物里出现
reasoning_price: 1.8000000000000001e-06（USD/token）。测完已把生产配置恢复为
6 条规则（备份 config.yaml.bak-billing-20261004-122151 为基准重建，
20 个源数量核对一致）。

Co-Authored-By: ModelRouter <noreply@modelrouter.dev>
(cherry picked from commit b93799a919)

internal/billing/compile.go

@@ -195,6 +195,18 @@ func applyRule(prices map[string]interface{}, rule *config.BillingRule, srcName
 			if t.CacheDiscount != nil {
 				entry["cache_discount"] = *t.CacheDiscount
 			}
+			// reasoning is optional and stays ABSENT (not nil, not 0) when the
+			// rule does not set it, which is what makes the billing plugin skip
+			// the repricing branch entirely. Writing an explicit 0 here would
+			// mean "thinking is free" and silently under-bill every request
+			// from a provider that bills it like ordinary output.
+			if strings.TrimSpace(t.Reasoning) != "" {
+				rp, err := usdPerM(t.Reasoning)
+				if err != nil {
+					return fmt.Errorf("model %q reasoning: %w", modelID, err)
+				}
+				entry["reasoning_price"] = rp
+			}
 			if rule.Peak != nil {
 				entry["peak"] = compilePeak(rule.Peak)
 			}

internal/billing/wildcard_test.go

@@ -86,3 +86,64 @@ func TestSpecificURLTokenRuleStillMarksItsSource(t *testing.T) {
 		t.Error("an unrelated source was marked priced by a specific-url rule")
 	}
 }
+
+// TestReasoningPriceIsOmittedUnlessSet pins the absence convention for the
+// thinking-token rate.
+//
+// A rule that says nothing about reasoning must produce a price entry with NO
+// reasoning_price key, so the billing plugin skips the repricing branch and
+// bills thinking at the ordinary output rate — which is what every provider
+// actually charges. Writing an explicit 0 instead would read as "thinking is
+// free" and silently under-bill every request from that model.
+func TestReasoningPriceIsOmittedUnlessSet(t *testing.T) {
+	sources := []config.Source{{Name: "up", BaseURL: "https://up.example/v1"}}
+
+	// Case 1: no reasoning declared → the key must not appear.
+	p1 := &config.BillingProfile{ID: "p", Rules: []config.BillingRule{{
+		URL: "https://up.example/v1", Mode: "token",
+		Models: map[string]config.BillingToken{
+			"m": {Prompt: "1.00", Completion: "2.00"},
+		},
+	}}}
+	prices, err := Compile(p1, sources)
+	if err != nil {
+		t.Fatal(err)
+	}
+	entry := prices["models"].(map[string]interface{})["m"].(map[string]interface{})
+	if _, present := entry["reasoning_price"]; present {
+		t.Errorf("reasoning_price present (%v) without a rule; the plugin would treat 0 as free",
+			entry["reasoning_price"])
+	}
+
+	// Case 2: declared → present, and converted to per-token like every other rate.
+	p2 := &config.BillingProfile{ID: "p", Rules: []config.BillingRule{{
+		URL: "https://up.example/v1", Mode: "token",
+		Models: map[string]config.BillingToken{
+			"m": {Prompt: "1.00", Completion: "2.00", Reasoning: "6.00"},
+		},
+	}}}
+	prices2, err := Compile(p2, sources)
+	if err != nil {
+		t.Fatal(err)
+	}
+	e2 := prices2["models"].(map[string]interface{})["m"].(map[string]interface{})
+	rp, ok := e2["reasoning_price"].(float64)
+	if !ok {
+		t.Fatalf("reasoning_price missing or wrong type: %#v", e2["reasoning_price"])
+	}
+	if rp != 6e-6 {
+		t.Errorf("reasoning_price = %v, want 6e-6 (USD per TOKEN)", rp)
+	}
+	// A malformed rate must be rejected, not silently ignored: an operator who
+	// typed "6 USD" expects either that value or an error, never a rule that
+	// looks applied and bills nothing.
+	p3 := &config.BillingProfile{ID: "p", Rules: []config.BillingRule{{
+		URL: "https://up.example/v1", Mode: "token",
+		Models: map[string]config.BillingToken{
+			"m": {Prompt: "1.00", Completion: "2.00", Reasoning: "six dollars"},
+		},
+	}}}
+	if _, err := Compile(p3, sources); err == nil {
+		t.Error("a non-numeric reasoning rate must fail the compile")
+	}
+}