feat: implement tiered billing expression evaluation and related functionality

- Added support for tiered billing expressions in the billing system.
- Introduced new types and functions for handling billing expressions, including caching and execution.
- Updated existing billing logic to accommodate tiered billing scenarios.
- Enhanced request handling to support incoming billing expression requests.
- Added tests for tiered billing functionality to ensure correctness.

setting/model_setting/claude_test.go

@@ -0,0 +1,60 @@
+package model_setting
+
+import (
+	"net/http"
+	"testing"
+)
+
+func TestClaudeSettingsWriteHeadersMergesConfiguredValuesIntoSingleHeader(t *testing.T) {
+	settings := &ClaudeSettings{
+		HeadersSettings: map[string]map[string][]string{
+			"claude-3-7-sonnet-20250219-thinking": {
+				"anthropic-beta": {
+					"token-efficient-tools-2025-02-19",
+				},
+			},
+		},
+	}
+
+	headers := http.Header{}
+	headers.Set("anthropic-beta", "output-128k-2025-02-19")
+
+	settings.WriteHeaders("claude-3-7-sonnet-20250219-thinking", &headers)
+
+	got := headers.Values("anthropic-beta")
+	if len(got) != 1 {
+		t.Fatalf("expected a single merged header value, got %v", got)
+	}
+	expected := "output-128k-2025-02-19,token-efficient-tools-2025-02-19"
+	if got[0] != expected {
+		t.Fatalf("expected merged header %q, got %q", expected, got[0])
+	}
+}
+
+func TestClaudeSettingsWriteHeadersDeduplicatesAcrossCommaSeparatedAndRepeatedValues(t *testing.T) {
+	settings := &ClaudeSettings{
+		HeadersSettings: map[string]map[string][]string{
+			"claude-3-7-sonnet-20250219-thinking": {
+				"anthropic-beta": {
+					"token-efficient-tools-2025-02-19",
+					"computer-use-2025-01-24",
+				},
+			},
+		},
+	}
+
+	headers := http.Header{}
+	headers.Add("anthropic-beta", "output-128k-2025-02-19, token-efficient-tools-2025-02-19")
+	headers.Add("anthropic-beta", "token-efficient-tools-2025-02-19")
+
+	settings.WriteHeaders("claude-3-7-sonnet-20250219-thinking", &headers)
+
+	got := headers.Values("anthropic-beta")
+	if len(got) != 1 {
+		t.Fatalf("expected duplicate values to collapse into one header, got %v", got)
+	}
+	expected := "output-128k-2025-02-19,token-efficient-tools-2025-02-19,computer-use-2025-01-24"
+	if got[0] != expected {
+		t.Fatalf("expected deduplicated merged header %q, got %q", expected, got[0])
+	}
+}