Support MySQL SET PERSIST/PERSIST_ONLY/LOCAL and dotted user variables

[renecannao]

ProxySQL PR #5893 surfaced ParserSQL SET grammar gaps while porting obsolete ProxySQL PR #5088 TAP coverage.

ParserSQL currently does not fully parse these MySQL SET forms through the SET AST:

SET persist character_set_server = 'utf8mb4';
SET persist_only innodb_buffer_pool_size = '1G';
SET PERSIST max_allowed_packet = 1073741824;
SET @@PERSIST.max_allowed_packet = 1073741824;
SET PERSIST_ONLY sql_mode = 'STRICT_TRANS_TABLES';
SET @@PERSIST_ONLY.sql_mode = 'STRICT_TRANS_TABLES';
SET LOCAL wait_timeout = 10;
SET LOCAL sql_mode := CONCAT(@@sql_mode, ',STRICT_TRANS_TABLES');
SET @'mix' := 1, LOCAL wait_timeout := 20;
SET @user.var := 7;

Expected behavior:

• Treat MySQL LOCAL, PERSIST, and PERSIST_ONLY as SET assignment scopes where valid.
• Handle comma-separated assignments that begin with those scope tokens.
• Normalize @@persist.var / @@persist_only.var as scoped system variables.
• Parse dotted single-at user variables like @user.var as a single variable target.

ProxySQL should consume this via a normal ParserSQL update, not a vendored local patch.

[coderabbitai]

🔗 Related PRs

#37 - fix(set): comma-separation after NAMES/CHARSET, ON keyword values, empty-result detection [merged]
#38 - fix(set): handle PostgreSQL SET TIME ZONE alias and multi-value lists [merged]
#39 - fix(set): coverage for ProxySQL set_parser_algorithm=3 audit (P1..P6) [merged]
#43 - fix(tokenizer): allow $ as PG identifier continuation char [merged]

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