Add length predicates for string length search

[tatematsu-k]

Summary

Implements predicates to search by string length as requested in #1492.

Changes

• Added length_eq, length_lt, length_lteq, length_gt, length_gteq predicates
• These predicates wrap the attribute in LENGTH() or CHAR_LENGTH() function based on the database adapter
• Added comprehensive tests for all length predicates

Usage

# Find records where name length is less than or equal to 4
Person.ransack(name_length_lteq: 4).result

# Find records where name length equals 10
Person.ransack(name_length_eq: 10).result

Database Support

• PostgreSQL/PostGIS: Uses CHAR_LENGTH()
• MySQL: Uses CHAR_LENGTH()
• SQLite and others: Uses LENGTH()

Closes #1492