feat(oracle): add production completion signals

[rebelice]

Summary

• Add parser-native Oracle completion context for Bytebase: candidates, prefix, scope, CTEs, and object intent.
• Cover SELECT, CTE/subquery scope, DML, DDL, PL/SQL, package/sequence member, database-link, prefix, and Oracle-specific completion scenarios.
• Document the expanded production scenario matrix for Oracle completion.

Test Plan

• go test ./oracle/parser -count=1
• Bytebase integration checked separately with go test ./backend/plugin/parser/plsql -count=1 after bumping to this omni commit.