Add string statistics measures (min_length, max_length, avg_length)

[stumelius]

Overview

For varchar/text columns, min and max are currently suppressed (they only apply to numeric and date columns). String columns get no shape information beyond distinct_count and not_null_proportion. Adding min_length, max_length, and avg_length fills this gap.

Motivation

String length statistics catch a class of data quality bugs that are otherwise invisible:

• Truncation bugs: a column that should hold 50-char values suddenly has max_length of 20
• Empty string vs null: not_null_proportion = 1.0 but min_length = 0 reveals unexpected empty strings
• Padding issues: unexpectedly uniform lengths suggest padding or fixed-width encoding
• Encoding problems: avg_length spikes when multi-byte characters are counted as bytes

Implementation notes

Simple cross-database implementation using length() / len():

-- default
min(length(col))
max(length(col))
avg(length(col))

-- SQL Server uses len()
min(len(col))
max(len(col))
avg(cast(len(col) as float))

Applied only when is_string_dtype(data_type) is true (mirrors how avg/median apply only to numeric types).

API design

Three new optional measures added to the default measure list:

exclude_measures: [min_length, max_length, avg_length]