Fix Oracle SQL*Plus command splitting

[rebelice]

Summary

• Preserve real Oracle SQL SET statements such as SET TRANSACTION, SET ROLE, and SET CONSTRAINT(S) as SQL segments.
• Avoid misclassifying SQL continuation lines like START WITH, CONNECT BY, CONNECT TO, and SET DEFAULT as SQL*Plus commands.
• Allow unambiguous SQLPlus commands, including PROMPT, SPOOL, SET DEFINE, SET DEF, SET SERVEROUT, and SQLPlus CONNECT, to terminate buffered SQL safely.

Test Plan

• go test ./oracle/parser -run 'TestSplitClassifiesUnambiguousSQLPlusCommandsAfterBufferedSQL|TestSplitDoesNotClassifySQLContinuationLinesAsSQLPlus|TestSplitClassifiesOracleSetStatementsAsSQL|TestSplitSQLPlusCommands|TestSplitClassifiesSQLPlusCommands' -count=1
• go test ./oracle ./oracle/parser -count=1
• git diff --check