refactor: Introduce pre-consume quota and unify relay handlers

This commit introduces a major architectural refactoring to improve quota management, centralize logging, and streamline the relay handling logic.

Key changes:
- **Pre-consume Quota:** Implements a new mechanism to check and reserve user quota *before* making the request to the upstream provider. This ensures more accurate quota deduction and prevents users from exceeding their limits due to concurrent requests.

- **Unified Relay Handlers:** Refactors the relay logic to use generic handlers (e.g., `ChatHandler`, `ImageHandler`) instead of provider-specific implementations. This significantly reduces code duplication and simplifies adding new channels.

- **Centralized Logger:** A new dedicated `logger` package is introduced, and all system logging calls are migrated to use it, moving this responsibility out of the `common` package.

- **Code Reorganization:** DTOs are generalized (e.g., `dalle.go` -> `openai_image.go`) and utility code is moved to more appropriate packages (e.g., `common/http.go` -> `service/http.go`) for better code structure.

service/http.go

@@ -0,0 +1,59 @@
+package service
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"net/http"
+	"one-api/common"
+	"one-api/logger"
+
+	"github.com/gin-gonic/gin"
+)
+
+func CloseResponseBodyGracefully(httpResponse *http.Response) {
+	if httpResponse == nil || httpResponse.Body == nil {
+		return
+	}
+	err := httpResponse.Body.Close()
+	if err != nil {
+		common.SysError("failed to close response body: " + err.Error())
+	}
+}
+
+func IOCopyBytesGracefully(c *gin.Context, src *http.Response, data []byte) {
+	if c.Writer == nil {
+		return
+	}
+
+	body := io.NopCloser(bytes.NewBuffer(data))
+
+	// We shouldn't set the header before we parse the response body, because the parse part may fail.
+	// And then we will have to send an error response, but in this case, the header has already been set.
+	// So the httpClient will be confused by the response.
+	// For example, Postman will report error, and we cannot check the response at all.
+	if src != nil {
+		for k, v := range src.Header {
+			// avoid setting Content-Length
+			if k == "Content-Length" {
+				continue
+			}
+			c.Writer.Header().Set(k, v[0])
+		}
+	}
+
+	// set Content-Length header manually BEFORE calling WriteHeader
+	c.Writer.Header().Set("Content-Length", fmt.Sprintf("%d", len(data)))
+
+	// Write header with status code (this sends the headers)
+	if src != nil {
+		c.Writer.WriteHeader(src.StatusCode)
+	} else {
+		c.Writer.WriteHeader(http.StatusOK)
+	}
+
+	_, err := io.Copy(c.Writer, body)
+	if err != nil {
+		logger.LogError(c, fmt.Sprintf("failed to copy response body: %s", err.Error()))
+	}
+}