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]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[codecov]

Codecov Report

❌ Patch coverage is 6.66667% with 28 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/dataset/write/merge_insert.rs	9.52%	19 Missing ⚠️
...ce/src/dataset/write/merge_insert/assign_action.rs	0.00%	6 Missing ⚠️
...lance/src/dataset/write/merge_insert/exec/write.rs	0.00%	2 Missing ⚠️
...nce/src/dataset/write/merge_insert/logical_plan.rs	0.00%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[wjones127]

Looks reasonable, but I have a minor change requested to deduplicate some logic.

[wjones127]

Looks good. Can merge once tests are passing.

[Ar-maan05]

CI is green and ready on my end. Let me know if anything else is needed before merge.