🧹 fix: harden request-body size handling and error unwrapping

Tighten oversized request handling across relay paths and make error matching reliable.

- Align `MAX_REQUEST_BODY_MB` fallback to `32` in request body reader and decompression middleware
- Stop ignoring `GetRequestBody` errors in relay retry paths; return consistent **413** on oversized bodies (400 for other read errors)
- Add `Unwrap()` to `types.NewAPIError` so `errors.Is/As` can match wrapped underlying errors
- `go test ./...` passes

common/gin.go

@@ -40,7 +40,7 @@ func GetRequestBody(c *gin.Context) ([]byte, error) {
 	}
 	maxMB := constant.MaxRequestBodyMB
 	if maxMB <= 0 {
-		maxMB = 64
+		maxMB = 32
 	}
 	maxBytes := int64(maxMB) << 20