Reconcile original and plan-derived LEADING hints in EXPLAIN FORMAT=hint

## Problem

`EXPLAIN FORMAT='hint'` currently appends table hints extracted from the original statement to hints derived from the physical plan. If the original statement already contains a `LEADING` hint, this can output both the original and plan-derived `LEADING` hints. Replaying that output rejects the duplicate `LEADING` hints instead of reproducing the plan.

## Expected behavior

Reconcile/deduplicate original-statement and plan-derived `LEADING` hints before formatting the result, so the generated hint text remains replayable.

## Scope

- Define precedence/reconciliation semantics when both sources contain `LEADING`.
- Avoid emitting duplicate/conflicting `LEADING` hints.
- Add round-trip tests for statements that already contain `LEADING`.

This is intentionally tracked separately from derived-table alias recovery and CTE hint handling.

Start from the EXPLAIN FORMAT='hint' path and trace where original-statement and physical-plan hints are combined. Define precedence for duplicate or conflicting LEADING hints, then add round-trip tests showing that statements with an existing LEADING hint produce replayable, non-duplicated output.

Written by the indexing model from the issue text.