feat: expose lossless MySQL user variables

[renecannao]

Summary

• add lossless MySQL user-variable and literal AST/token support
• preserve exact source spelling for safe downstream replay
• classify user-variable usage as absent, read-only, or unsafe/unknown
• enforce full-input parsing and harden malformed/comment edge cases, including MariaDB executable comments

Why

ProxySQL currently falls back to hostgroup locking for client statements such as:

SET @browser_lang ='en-US', @browser_time ='2026-08-11 18:11:12', @browser_timezone ='GMT+2', @ip_address ='167.235.198.244'

The downstream consumer needs typed assignments and the exact literal spelling before it can safely track and replay these values. Reconstructing that information from normalized SQL would lose literal semantics, so ParserSQL is the source of truth.

Details

• supports string, integer, decimal, hexadecimal, bit, and NULL literal nodes
• supports direct +/- signs for numeric literals
• exposes canonical user-variable identity together with lossless token/source ranges
• detects user-variable reads and unsafe assignments without raw-text semantic guesses
• rejects partial/malformed input conservatively
• preserves existing emitter and digest behavior, including quoted user-variable forms

Validation

• make test: 1,333 tests run; 1,296 passed; 37 backend-dependent tests skipped
• make build-corpus-test: passed
• downstream ProxySQL ParserSQL adapter: 199/199 unit assertions
• downstream legacy ParserSQL SET comparison: 494/494 assertions

Downstream

Companion ProxySQL PR: sysown/proxysql#6043

That PR vendors the archive produced from commit db9d464c334eaba236bea16a6e9eb6efa7c3a863 and uses these typed results for bounded user-variable tracking and backend replay.

[coderabbitai]

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