[KV] Add retract support for Aggregation Merge Engine

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.

### Motivation

The Aggregation Merge Engine currently supports upsert and deleteoperations. However, there is no way to **reverse a previously applied aggregation contribution** without deleting the entire row.

Consider an e-commerce scenario: a table aggregates total revenue per shop using `SUM`. When a refund occurs, the system needs to subtract the refunded amount from the running total. Today, this requires the Flink compute layer to maintain full aggregation state — defeating the purpose of pushing aggregation down to the storage engine.

### Solution

Introduce a **retract** operation that computes the inverse of a previous aggregation at the storage layer:
- `SUM`: subtracts the retracted value
- `PRODUCT`: divides by the retracted value
- `LAST_VALUE` / `LAST_VALUE_IGNORE_NULLS`: clears to null Functions that cannot be mathematically reversed (`MAX`, `MIN`, `FIRST_VALUE`, `LISTAGG`, etc.) do not support retract.

### Anything else?

_No response_

### Willingness to contribute

- [x] I'm willing to submit a PR!

Start by locating the Aggregation Merge Engine and reviewing its existing upsert and delete paths. Trace how SUM, PRODUCT, LAST_VALUE, and LAST_VALUE_IGNORE_NULLS are implemented, along with the unsupported MAX, MIN, FIRST_VALUE, and LISTAGG functions. Done means retract behavior is covered for supported functions and unsupported functions remain excluded.

Written by the indexing model from the issue text.