feat(rust): support datafusion expressions for merge insert predicates

[Ar-maan05]

Description

This PR adds support for passing logical DataFusion expressions (datafusion_expr::Expr) directly to the merge insert query planner for conditional updates (WhenMatched::UpdateIfExpr). Currently, matched update predicates only accept SQL strings which are parsed at plan creation time, introducing serialize-deserialize round-trip overhead when callers build expressions programmatically.

Key Changes

• WhenMatched Enum: Added the UpdateIfExpr(datafusion_expr::Expr) variant and the associated constructor helper WhenMatched::update_if_expr(expr: Expr).
• Query Planner (merge_insert.rs): Updated plan generation to optimize the DataFusion logical expression and convert it directly to a physical expression.
• Physical & Logical Layouts: Handled the new variant exhaustively in assign_action.rs (action determination), exec/write.rs (DisplayAs), and logical_plan.rs (Explain formatting).

Verification

• Verified compile-safety and workspace formatting checks.
• Verified via downstream integration tests in lancedb.

Closes #6861

[claude]

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