fix: Oracle explain_as_text() implementation

## 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