fix(deparser): preserve parentheses around binary expressions in unary operators

[pyramation]

fix(deparser): preserve parentheses around binary expressions in unary operators

Summary

Fixes #285. The unary operator branch in the A_Expr handler did not wrap binary sub-expressions in parentheses, causing expressions like -(a - b) to deparse as - a - b — silently changing operator precedence and producing mathematically incorrect results.

This also affected TypeCast conversions: (-(a - b))::numeric became CAST(- a - b AS numeric) instead of CAST(- (a - b) AS numeric).

Fix: When a unary AEXPR_OP has an rexpr that is itself a binary AEXPR_OP (i.e., it has a lexpr), wrap the visited sub-expression in parentheses. Unary-inside-unary (e.g., - -x) is correctly left unwrapped since the inner expression has no lexpr.

Two test fixtures added for the exact SQL from the issue report.

Review & Testing Checklist for Human

• Verify the parenthesization condition is complete: The fix only triggers for AEXPR_OP kind with a lexpr. Other A_Expr kinds (AEXPR_IN, AEXPR_BETWEEN, AEXPR_LIKE, etc.) inside a unary operator are not parenthesized by this change. Confirm these don't produce ambiguous output (they likely don't since they use keyword syntax, but worth a quick mental check).
• Spot-check edge cases manually: Try parse → deparse on expressions like -(a + b * c), -(a IS NOT NULL), - -x, (-x)::int to verify no over/under-parenthesization.
• Confirm fixture numbering didn't collide: The new fixtures shifted issue-18 from the EXCLUDE WHERE test to the new unary-minus test. The EXCLUDE WHERE test moved to issue-20. Verify no other code depends on specific fixture numbers.

Notes

• All 290 deparser test suites pass (710 tests, 424 snapshots) with no regressions.
• The fix is intentionally narrow — only binary AEXPR_OP inside unary AEXPR_OP — to minimize risk of over-parenthesization.

Link to Devin run: https://app.devin.ai/sessions/3f7a5f172eff49b388c44b148691911c
Requested by: @pyramation

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