[zoilomora]

Prevent inconsistent log field types from causing Elasticsearch indexing conflicts.

• Serialize exception codes as strings and map them as keyword, supporting both numeric codes and values such as HY000.
• Preserve UTF-8 characters when truncating payload strings, keeping the existing 128-byte preview limit.
• Replace invalid UTF-8 and preserve serializable fields when JSON encoding encounters unsupported values, preventing boolean false payloads.
• Declare ext-mbstring as a dependency and fix coding standard violations.

Validation: 41 tests and 119 assertions pass. PHPCS passes for all source files; the existing dependency configuration deprecation remains.

Deployment: apply the updated Elasticsearch template. Existing indices retain their current mappings and require migration separately.