MDEV-39143 Replicate MySQL binary JSON to a MariaDB slave

[plampio]

Let a MariaDB slave apply ROW-format binlog events from a MySQL master that contain JSON columns, both as full documents and as MySQL's partial (diff) JSON updates (PARTIAL_UPDATE_ROWS_EVENT).

Full documents reuse the slave's existing type-conversion path: the master's binlog type code 245 is mapped to the read-only mysql_json plugin's handler (table_def::field_type_handler()), and Field_longstr::rpl_conv_type_from() accepts it as a lossless conversion into a native JSON column or a lossy one into plaintext/blob, gated by slave_type_conversions as usual.

Partial updates are new. unpack_row() recognises PARTIAL_UPDATE_ROWS_EVENT, reads the after-image's value_options/partial_bits header, and for each column marked partial decodes and applies its Json_diff vector directly against the row's before-image via MariaDB's own JSON_REPLACE/JSON_INSERT/JSON_ARRAY_INSERT/JSON_REMOVE, bypassing the normal conversion-table copy step (there is no final value to copy until the diffs are applied).

An INSERT diff selects JSON_ARRAY_INSERT or JSON_INSERT based on the diff path's last leg, per MySQL's own semantics. A diff is refused, stopping the SQL thread, when the path is a from-the-end array index ('[last-N]'): MariaDB's JSON_ARRAY_INSERT resolves that case differently from MySQL, which would otherwise silently diverge from the source. Decoding a diff's JSONB value now uses Field_blob:: set_ptr() rather than Field::store(), since store() validated the raw bytes as utf8mb4 and rejected any value needing a multi-byte JSONB length (i.e. longer than 127 bytes).

Also improves the missing-plugin diagnostic: a JSON column arriving without mysql_json installed on the slave now names the plugin and the required INSTALL SONAME, instead of reporting an unknown column type.

[CLAassistant]

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