feat: support BackedEnum values in database binds

[memleakd]

Description

This PR adds support for PHP BackedEnum values in database escaping, query bindings, and escaped Query Builder values.

It was inspired by @maniaba's forum post and #10223.

For apps that use native PHP enums for domain values, this makes database code a bit nicer to write:

$db->table('users')
    ->where('status', UserStatus::Active)
    ->get();

CodeIgniter will use the enum's backing value when escaping it, so string-backed enums are treated like strings and int-backed enums are treated like integers.

This avoids repeating ->value at every query call site, while keeping raw SQL and escape-disabled values in the caller's control.

The implementation is intentionally small: enum cases are unwrapped during database escaping, so regular query bindings and escaped Query Builder values use the same existing path.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide