feat: support conditional delete in merge-insert when_matched clause

Problem

WhenMatched supports conditional update through UpdateIf and UpdateIfExpr, but delete is only unconditional. Conditional delete exists solely on WhenNotMatchedBySource::DeleteIf, and the Python binding when_matched_delete() takes no condition.

This makes it impossible to express "delete the matched target row only when a condition holds" in a single merge-insert. Examples are tombstone-driven deletes (source.deleted = true) and stale-delete guards (source.last_update > target.last_update). The current workaround is a separate scan and delete pass, which is not atomic with the merge.

Expected behavior

Rust:

let job = dataset.merge_insert(&["id"])
    .when_matched(WhenMatched::delete_if(&dataset, "source.deleted = true")?)
    ...

Python, mirroring when_matched_update_all(condition):

dataset.merge_insert("id") \
    .when_matched_delete("source.deleted = true") \
    .when_not_matched_insert_all() \
    .execute(new_data)

The condition supports source. and target. qualified references like UpdateIf does. Matched rows failing the condition are left untouched. Bare when_matched_delete() keeps its current unconditional behavior.

Proposed implementation

• Add WhenMatched::DeleteIf(String) and DeleteIfExpr(Expr) variants mirroring UpdateIf and UpdateIfExpr, with parsing deferred because the two execution paths resolve source. and target. references against different schemas (relation-qualified columns on the standard plan, combined-schema struct fields on the indexed path).
• Fast path: new CASE arms in assign_action.rs emitting Action::Delete when a row is matched and the condition is true, falling through to Action::Nothing otherwise. Delete-only plans keep using DeleteOnlyMergeInsertExec unchanged since the condition is baked into the action assignment.
• Slow (indexed) path: reuse the Merger's existing match_filter_expr compilation and apply the mask to matched rows before recording deletions.
• Python: optional condition parameter on the existing when_matched_delete, fully backward compatible.

Suggested coverage

• Rust tests for both execution paths (the fast-path plan and the indexed Merger path), asserting num_deleted_rows counts only condition-true matches and surviving rows are unmodified. Error case for non-boolean conditions.
• Python test extending test_merge_insert_when_matched_delete with conditional cases.

Follow-up (out of scope here): Java MergeInsertParams.withMatchedDeleteIf(String) mirroring withNotMatchedBySourceDeleteIf.

Start with the WhenMatched variants, assign_action.rs, and the existing Merger match_filter_expr path; compare the fast-path plan with the indexed path. Extend the Rust coverage for both paths and the non-boolean error, then extend test_merge_insert_when_matched_delete for Python. Done means conditional matches alone are deleted, other matched rows remain unchanged, and unconditional deletion still works.