Separate MySQL 5/8 features via replaceable driver architecture

[Copilot]

Refactored monolithic driver into version-specific implementations to enable coexistence of MySQL 5 and MySQL 8 features with swappable driver classes.

Architecture

Factory pattern with automatic version detection:

• Gratan::Driver now factory method that detects MySQL version and instantiates appropriate driver
• Gratan::Driver::Base - common functionality (276 lines)
• Gratan::Driver::MySQL5 - MySQL 5.6 (no SHOW CREATE USER)
• Gratan::Driver::MySQL57 - MySQL 5.7 (SHOW CREATE USER support)
• Gratan::Driver::MySQL8 - MySQL 8.0+ (ALTER USER, no PASSWORD() function)

Version-Specific Behavior

Feature	MySQL 5.6	MySQL 5.7	MySQL 8.0
SHOW CREATE USER	❌	✅	✅
Password management	SET PASSWORD	SET PASSWORD	ALTER USER
PASSWORD() function	✅	✅	❌ (removed)

Usage

Automatic detection (default):

client = Gratan::Client.new(host: 'localhost', username: 'root')
# Detects version, instantiates MySQL5/MySQL57/MySQL8 driver

Manual override:

client = Gratan::Client.new(
  host: 'localhost',
  username: 'root',
  driver_class: Gratan::Driver::MySQL8
)

Changes

• Reduced lib/gratan/driver.rb from 287 to 28 lines (now factory)
• Extracted common logic to Base class
• Version-specific implementations in separate files
• Added 16 tests covering factory logic and integration
• 100% backward compatible - no API changes

Net: +540 lines across 12 files

Original prompt

mysql8 系の機能と同居するために，mysql5 の機能を分離できる状態にしたい．実装クラスを入れ替えられるようにしたい

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