ENH: Add totality validation to merge method about pandas

[z3rone]

To maybe add a common use case. Here the goal is to add the biological domain to the favorite animal of certain people:

import pandas as pd

# Create the first DataFrame with person names and favorite animals
df1_data = {
    'Person': ['John', 'Emma', 'Alex','Darleen'],
    'Animal': ['Dog', 'Spider', 'Snake','Cat']
}
df1 = pd.DataFrame(df1_data)

# Create the second DataFrame with mapping of animals to biological class
df2_data = {
    'Animal': ['Dog', 'Snake', 'Cat'],
    'Biological_Class': ['Mammal', 'Reptile', 'Mammal']
}
df2 = pd.DataFrame(df2_data)

# Merge the DataFrames on the 'Animal' column
merged_df = pd.merge(
    df1,
    df2,
    on='Animal',
    validate='m:1'
)

The merged_df will lack the favorite animal of Emma, as 'Spider' has no class defined in df2. With the proposed feature validate could be set to m:1+left_total. This would raise an error as not all keys from the left df1 are contained in the right df2.