feat: 实现统一模型 ID 机制

实现统一模型 ID 格式 (provider_id/model_name)，支持跨协议模型标识和 Smart Passthrough。

核心变更：
- 新增 pkg/modelid 包：解析、格式化、校验统一模型 ID
- 数据库迁移：models 表使用 UUID 主键 + UNIQUE(provider_id, model_name) 约束
- Repository 层：FindByProviderAndModelName、ListEnabled 方法
- Service 层：联合唯一校验、provider ID 字符集校验
- Conversion 层：ExtractModelName、RewriteRequestModelName/RewriteResponseModelName 方法
- Handler 层：统一模型 ID 路由、Smart Passthrough、Models API 本地聚合
- 新增 error-responses、unified-model-id 规范

测试覆盖：
- 单元测试：modelid、conversion、handler、service、repository
- 集成测试：统一模型 ID 路由、Smart Passthrough 保真性、跨协议转换
- 迁移测试：UUID 主键、UNIQUE 约束、级联删除

OpenSpec：
- 归档 unified-model-id 变更到 archive/2026-04-21-unified-model-id
- 同步 11 个 delta specs 到 main specs
- 新增 error-responses、unified-model-id 规范文件

backend/pkg/modelid/model_id.go

@@ -0,0 +1,63 @@
+package modelid
+
+import (
+	"errors"
+	"regexp"
+	"strings"
+)
+
+var providerIDRegex = regexp.MustCompile(`^[a-zA-Z0-9_]+$`)
+
+// ParseUnifiedModelID 将 "provider_id/model_name" 格式的字符串解析为 providerID 和 modelName
+// 在第一个 "/" 处分割，model_name 可以包含 "/"
+func ParseUnifiedModelID(id string) (providerID, modelName string, err error) {
+	if id == "" {
+		return "", "", errors.New("统一模型 ID 不能为空")
+	}
+
+	parts := strings.SplitN(id, "/", 2)
+	if len(parts) != 2 {
+		return "", "", errors.New("统一模型 ID 格式错误，缺少分隔符 \"/\"")
+	}
+
+	providerID = parts[0]
+	modelName = parts[1]
+
+	if providerID == "" {
+		return "", "", errors.New("provider_id 不能为空")
+	}
+	if modelName == "" {
+		return "", "", errors.New("model_name 不能为空")
+	}
+
+	if !providerIDRegex.MatchString(providerID) {
+		return "", "", errors.New("provider_id 仅允许字母、数字、下划线")
+	}
+
+	return providerID, modelName, nil
+}
+
+// FormatUnifiedModelID 将 providerID 和 modelName 组合格式化为统一模型 ID
+func FormatUnifiedModelID(providerID, modelName string) string {
+	return providerID + "/" + modelName
+}
+
+// ValidateProviderID 校验 providerID 仅包含字母、数字、下划线，长度 1-64
+func ValidateProviderID(id string) error {
+	if id == "" {
+		return errors.New("provider_id 不能为空")
+	}
+	if len(id) > 64 {
+		return errors.New("provider_id 长度不能超过 64 个字符")
+	}
+	if !providerIDRegex.MatchString(id) {
+		return errors.New("provider_id 仅允许字母、数字、下划线")
+	}
+	return nil
+}
+
+// IsValidUnifiedModelID 判断字符串是否为合法的统一模型 ID 格式
+func IsValidUnifiedModelID(id string) bool {
+	_, _, err := ParseUnifiedModelID(id)
+	return err == nil
+}

backend/pkg/modelid/model_id_test.go

@@ -0,0 +1,96 @@
+package modelid
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestParseUnifiedModelID_StandardFormat(t *testing.T) {
+	providerID, modelName, err := ParseUnifiedModelID("openai/gpt-4")
+	assert.NoError(t, err)
+	assert.Equal(t, "openai", providerID)
+	assert.Equal(t, "gpt-4", modelName)
+}
+
+func TestParseUnifiedModelID_ModelNameWithSlashes(t *testing.T) {
+	providerID, modelName, err := ParseUnifiedModelID("azure/accounts/org-123/models/gpt-4")
+	assert.NoError(t, err)
+	assert.Equal(t, "azure", providerID)
+	assert.Equal(t, "accounts/org-123/models/gpt-4", modelName)
+}
+
+func TestParseUnifiedModelID_MissingSeparator(t *testing.T) {
+	_, _, err := ParseUnifiedModelID("gpt-4")
+	assert.Error(t, err)
+}
+
+func TestParseUnifiedModelID_EmptyString(t *testing.T) {
+	_, _, err := ParseUnifiedModelID("")
+	assert.Error(t, err)
+}
+
+func TestParseUnifiedModelID_OnlySeparator(t *testing.T) {
+	tests := []string{"/model", "provider/", "/"}
+	for _, tc := range tests {
+		_, _, err := ParseUnifiedModelID(tc)
+		assert.Error(t, err, "输入 %q 应返回错误", tc)
+	}
+}
+
+func TestParseUnifiedModelID_InvalidProviderID(t *testing.T) {
+	tests := []string{
+		"open-ai/gpt-4",
+		"open.ai/gpt-4",
+		"供应商/gpt-4",
+		"open ai/gpt-4",
+	}
+	for _, tc := range tests {
+		_, _, err := ParseUnifiedModelID(tc)
+		assert.Error(t, err, "providerID 含非法字符 %q 应返回错误", tc)
+	}
+}
+
+func TestFormatUnifiedModelID(t *testing.T) {
+	assert.Equal(t, "openai/gpt-4", FormatUnifiedModelID("openai", "gpt-4"))
+	assert.Equal(t, "anthropic/claude-3-opus", FormatUnifiedModelID("anthropic", "claude-3-opus"))
+}
+
+func TestValidateProviderID_Valid(t *testing.T) {
+	validIDs := []string{"openai", "deep_seek", "provider01", "OpenAI"}
+	for _, id := range validIDs {
+		assert.NoError(t, ValidateProviderID(id), "%q 应校验通过", id)
+	}
+}
+
+func TestValidateProviderID_InvalidChars(t *testing.T) {
+	invalidIDs := []string{"open-ai", "open.ai", "open ai", "供应商", "open/ai"}
+	for _, id := range invalidIDs {
+		assert.Error(t, ValidateProviderID(id), "%q 应校验失败", id)
+	}
+}
+
+func TestValidateProviderID_Empty(t *testing.T) {
+	assert.Error(t, ValidateProviderID(""))
+}
+
+func TestValidateProviderID_TooLong(t *testing.T) {
+	longID := strings.Repeat("a", 65)
+	assert.Error(t, ValidateProviderID(longID))
+
+	exactly64 := strings.Repeat("a", 64)
+	assert.NoError(t, ValidateProviderID(exactly64))
+}
+
+func TestIsValidUnifiedModelID(t *testing.T) {
+	assert.True(t, IsValidUnifiedModelID("openai/gpt-4"))
+	assert.True(t, IsValidUnifiedModelID("anthropic/claude-3-opus-20240229"))
+	assert.True(t, IsValidUnifiedModelID("azure/accounts/org/models/gpt-4"))
+
+	assert.False(t, IsValidUnifiedModelID(""))
+	assert.False(t, IsValidUnifiedModelID("gpt-4"))
+	assert.False(t, IsValidUnifiedModelID("open-ai/gpt-4"))
+	assert.False(t, IsValidUnifiedModelID("/model"))
+	assert.False(t, IsValidUnifiedModelID("provider/"))
+}