feat: 配置 golangci-lint 静态分析并修复存量违规

- 新增 backend/.golangci.yml 配置 12 个 linter（forbidigo、errorlint、errcheck、staticcheck、revive、gocritic、gosec、bodyclose、noctx、nilerr、goimports、gocyclo）
- 新增 lefthook.yml 配置 pre-commit hook 自动运行 lint
- 修复存量代码违规：errors.Is/As 替换、zap.Error 替换、import 排序、errcheck 修复
- 更新 README 补充编码规范说明
- 归档 backend-code-lint 变更

backend/internal/conversion/anthropic/stream_encoder_test.go

@@ -80,7 +80,8 @@ func TestStreamEncoder_ContentBlockStart_Text(t *testing.T) {
 			break
 		}
 	}
-	cb := payload["content_block"].(map[string]any)
+	cb, ok := payload["content_block"].(map[string]any)
+	require.True(t, ok)
 	assert.Equal(t, "text", cb["type"])
 }
 
@@ -107,7 +108,8 @@ func TestStreamEncoder_ContentBlockStart_ToolUse(t *testing.T) {
 			break
 		}
 	}
-	cb := payload["content_block"].(map[string]any)
+	cb, ok := payload["content_block"].(map[string]any)
+	require.True(t, ok)
 	assert.Equal(t, "tool_use", cb["type"])
 	assert.Equal(t, "toolu_1", cb["id"])
 	assert.Equal(t, "search", cb["name"])
@@ -131,7 +133,8 @@ func TestStreamEncoder_ContentBlockStart_Thinking(t *testing.T) {
 			break
 		}
 	}
-	cb := payload["content_block"].(map[string]any)
+	cb, ok := payload["content_block"].(map[string]any)
+	require.True(t, ok)
 	assert.Equal(t, "thinking", cb["type"])
 }
 
@@ -173,7 +176,8 @@ func TestStreamEncoder_MessageDelta_WithStopReason(t *testing.T) {
 			break
 		}
 	}
-	delta := payload["delta"].(map[string]any)
+	delta, okd := payload["delta"].(map[string]any)
+	require.True(t, okd)
 	assert.Equal(t, "end_turn", delta["stop_reason"])
 }
 
@@ -199,7 +203,8 @@ func TestStreamEncoder_MessageDelta_WithUsage(t *testing.T) {
 			break
 		}
 	}
-	u := payload["usage"].(map[string]any)
+	u, oku := payload["usage"].(map[string]any)
+	require.True(t, oku)
 	assert.Equal(t, float64(88), u["output_tokens"])
 }