[BUG] MERGE silently rewrites illegal WHEN NOT MATCHED clause pairings into the opposite semantics

[fudianchn]

Failing SQL Feature:

MERGE accepts illegal WHEN NOT MATCHED side / clause pairings and silently rewrites them into semantically different, legal SQL. SQL Server and BigQuery (the dialects providing BY TARGET / BY SOURCE) reject these pairings as syntax errors.

Full behavior matrix on master f41c0b8:

Clause pairing	Legal?	Result on master
WHEN MATCHED THEN UPDATE/DELETE	yes	accepted, round-trips
WHEN MATCHED THEN INSERT	no	rejected
WHEN NOT MATCHED THEN INSERT	yes	accepted, round-trips
WHEN NOT MATCHED THEN UPDATE/DELETE	no	accepted, deparses as WHEN MATCHED (condition inverted)
WHEN NOT MATCHED BY TARGET THEN INSERT	yes	accepted, round-trips
WHEN NOT MATCHED BY TARGET THEN UPDATE/DELETE	no	accepted, deparses as WHEN MATCHED (condition inverted)
WHEN NOT MATCHED BY SOURCE THEN UPDATE/DELETE	yes	accepted, round-trips
WHEN NOT MATCHED BY SOURCE THEN INSERT	no	accepted, deparses as WHEN NOT MATCHED (BY SOURCE dropped, different row set)

The parser therefore silently changes the meaning of the statement instead of failing, which is the worst failure mode for auditing / firewall tools built on the parser.

Root cause: MergeWhenNotMatched (JSqlParserCC.jjt:4472) accepts any clause for any side, while the AST can only represent the legal pairings: MergeUpdate:82 / MergeDelete:55 render WHEN MATCHED for any side other than SOURCE, and MergeInsert:77 always renders WHEN NOT MATCHED. Introduced with the BY TARGET / BY SOURCE support from #2421 / #2453, which validated the six legal combinations but not the illegal ones.

SQL Example:

// accepted on master, silently rewritten:
CCJSqlParserUtil.parse("MERGE INTO t USING s ON t.id = s.id"
        + " WHEN NOT MATCHED BY TARGET THEN UPDATE SET a = 1").toString()
// => MERGE INTO t USING s ON t.id = s.id WHEN MATCHED THEN UPDATE SET a = 1

CCJSqlParserUtil.parse("MERGE INTO t USING s ON t.id = s.id"
        + " WHEN NOT MATCHED BY SOURCE THEN INSERT (a) VALUES (1)").toString()
// => MERGE INTO t USING s ON t.id = s.id WHEN NOT MATCHED THEN INSERT (a) VALUES (1)

Software Information:

• JSqlParser version: 5.4-SNAPSHOT (master f41c0b8)
• Database: SQL Server / BigQuery (WHEN NOT MATCHED [BY TARGET|BY SOURCE] dialects)