Plan Iceberg WriteDelta in the split write path and keep it on the JVM DeltaWriter #6692

Part of #6240. Related: #5122, #5649.

What is the problem?

On Spark 3.5+, an Iceberg merge-on-read DELETE, UPDATE or MERGE writes through Spark's WriteDelta command. Comet's split Iceberg write path (IcebergWrite + IcebergCommit) does not recognise it today, so those writes either stay on Spark's stock plan or are caught by checks written for copy-on-write and append writes. There is no explicit, tested statement of what happens to a position-delta write.

The native position-delta writer in #6240 needs this first. Before native code can route data rows and position deletes, Comet must recognise WriteDelta, extract what a delta write needs (the operation column and the data, row-id and metadata projections), and keep running Iceberg's JVM DeltaWriter for it. That makes later native work a swap of the writer behind a known dispatch point rather than a planner change.

Proposed solution

The first item of the incremental split in #6240, with behaviour unchanged for users:

• Recognise Iceberg WriteDelta on Spark 3.5+; Spark 3.4 keeps the stock plan.
• Carry only what the JVM delta path needs: the operation and Spark's projection state, using Spark's RowDeltaUtils operation constants instead of copies of the numeric ids.
• Run position-delta rows through Iceberg's JVM DeltaWriter. The native Iceberg writer stays out of the delta path (CometIcebergWriteExec is never used for position-delta rows).
• Keep row-level write summaries, writer reporting, AQE behaviour and failure cleanup the same as the stock Iceberg path.
• Fall back cleanly when the version-specific logical-plan extraction cannot be done, with a stated reason.

Scope

In scope: planning, extraction and JVM dispatch for position-delta writes, with tests.

Out of scope (later items in #6240): a native delta operator or protobuf, a native position-delete writer or file layout contract, loading previous deletes, and a native feature flag. No Rust changes or dependencies.

Acceptance criteria

• DELETE, UPDATE and MERGE results match the stock Iceberg JVM path.
• A position-delta write plans without CometIcebergWriteExec.
• Copy-on-write and merge-on-read MERGE plans match the stock path with AQE on and off.
• Row-level snapshot summaries match where Spark exposes them.
• Writer reporting classifies the intercepted JVM WriteDelta write.
• Task commit location extraction covers position-delta data and delete files.
• Unsupported shapes fall back with a reason.