[6.x] Fix nocache database driver failing on MySQL with invalid UTF-8

[jasonvarga]

Fixes #14504.

Problem

Since 6.10.0, DatabaseSession::cacheRegion() stores serialize($region) directly into the region LONGTEXT column of nocache_regions. PHP's serialize() emits binary content around protected/private property markers (\x00*\x00, \x00ClassName\x00) that is not valid UTF-8. MySQL's utf8mb4 charset rejects these sequences:

SQLSTATE[HY000]: General error: 1366 Incorrect string value: '\xDB\xE2\x88\xB2...' for column 'region'

PostgreSQL and SQLite don't enforce UTF-8 on text columns, so they're unaffected.

Fix

Wrap the serialized string with base64_encode/base64_decode in DatabaseSession so the value written to MySQL is always pure ASCII. This matches the workaround documented in the issue.

This was chosen over changing the column type to binary/blob because a schema change would require a new migration for every existing install that has already run statamic:nocache:migration. The ~33% base64 size overhead is negligible against a LONGTEXT column (4 GB ceiling) and typical region payload sizes.

The cache-backed Session path is unchanged — Redis/memcached/file stores don't enforce UTF-8.