Avoid duplicate bucket ID projection in native write paths

## Overview
Native write paths in Spark 3.2/3.3 and the ClickHouse MergeTree writer
recompute the `__bucket_value__` expression even when a precomputed
attribute already exists. This adds unnecessary overhead and complicates
downstream projections.

## Steps to Reproduce
- Write bucketed data using the native writer.
- Inspect the execution plan; the bucket ID is projected multiple times
instead of reusing a single attribute.

## Expected Behavior
The bucket ID should be computed once (e.g., in an initial `ProjectExec`)
and reused by subsequent stages.

## Actual Behavior
Every stage re-evaluates the bucket expression, leading to redundant
projections and performance overhead.

## Impact
- Increased CPU time for bucketed writes.
- Harder-to-read execution plans with repetitive projections.

## Proposed Fix
- Guard projections in Spark 3.2/3.3 shims and ClickHouse MergeTree writer
so they append `__bucket_value__` only when missing.
- Store the computed bucket ID in an attribute and reuse it downstream.

Start by reading the Spark 3.2/3.3 shims and the ClickHouse MergeTree writer, then reproduce the native bucketed write and inspect its execution plan. Trace where __bucket_value__ is projected and compare the paths that already have a precomputed attribute. Done means the bucket expression is projected once and reused downstream without duplicate projections.

Written by the indexing model from the issue text.