fix(sql): read 'a\\' the way PostgreSQL and SQLite do in the splitter, MCP guard and destructive-query check

[ZhuchkaTriplesix]

Found while checking a review of the lexers: SqlStatementSplitter had its own copy of the quote/comment loop and always treated \' as an escaped quote (MySQL). SqlLexicalScanner has been dialect-aware since #1566. In PostgreSQL and SQLite a backslash is an ordinary character, so for

SELECT 'a\'; SELECT pg_terminate_backend(1); --'

the splitter and stripCommentsAndStrings saw one statement with a long literal, while the server runs the second statement.

Affected, all by reading the code (not reproduced against a live server):

• McpSqlGuard (single_statement, _forbiddenFunctions, isMutatingSqlStatement): the first layer could be passed by text hidden after such a literal. The read-only session is the second layer and stays; it does not stop functions like pg_terminate_backend.
• DestructiveSqlDetector.inspect (the confirm dialog before a destructive query) and the SQL workspace's statement ranges.

Change

• SqlStatementSplitter.spans is built on SqlLexicalScanner.segments (a ; only counts in code segments), which removes the duplicated loop. New backslashEscapes parameter, default true: MySQL and unknown dialects behave exactly as before.
• DestructiveSqlDetector.splitStatements / stripCommentsAndStrings / inspect, isMutatingSqlStatement and containsMutatingSql take the same parameter; E'...' strings keep their escape in every dialect.
• SqlDialect.backslashEscapesQuotes says it once; the MCP guard and the SQL workspace pass it.

Not covered: auditSqlExecution (mutation_audit_recorder.dart) still calls containsMutatingSql without a dialect, so a hidden mutation may be missing from the audit trail; it needs the connection type mapped to a dialect and is a follow-up.

Tests: the sample above for the splitter, the guard (PostgreSQL and SQLite refused; MySQL one literal; E'...' allowed), the destructive check and the classifier, plus a script over the isolate threshold splitting the same through spansAsync.