fix: Oracle explain_as_text() implementation

[dtsong]

Problem

oracle.py:107 raises NotImplementedError("Explain not yet implemented in Oracle"). This blocks query plan debugging for Oracle users.

Fix

Oracle supports EXPLAIN PLAN FOR <sql> followed by SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY).

Acceptance Criteria

• explain_as_text() returns Oracle query plan
• Unit test covers SQL generation

[github-actions]

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