opt: simplify COALESCE using NOT NULL column derivation

[lohitkolluri]

Fixes #103596

Adds normalization rules that simplify COALESCE expressions by dropping provably-non-null operands, using NOT NULL column information derived from relational properties and filter constraints. For example:

COALESCE(NULL, x, y) → COALESCE(x, y)
COALESCE(x::INT, y) → x::INT (if x is provably non-null)

The simplification works at two levels: (1) a recursive walk over scalar expressions that catches nested COALESCE and sub-expressions, and (2) filter-level analysis that derives NOT NULL columns from other filter items' constraints (e.g., b > 0 AND COALESCE(b, c) = 1 → b = 1).

Epic: none
Release note (sql change): The optimizer can now simplify COALESCE expressions by dropping operands that are provably non-null, including from filter conditions using cross-filter NOT NULL derivation.

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[lohitkolluri]

hey @DrewKimball @mw5h @michae2 sorry for the trouble with my previous PR (#170858) — it had to be closed due to a mistake I made (shallow clone + push caused file bloat). I've recreated it cleanly from master with only the 5 intended implementation files and all the review feedback y'all raised is incorporated (function renames, removed unnecessary helpers, inlined optgen patterns, pointer-equality reuse for unchanged items)

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[lohitkolluri]

@michae2 Hi! Just following up on this PR to see if you've had a chance to review it. No rush, just wanted to check in. Thanks!