[MINOR][CH] Correct misleading Spark 3.2 comment in ReadBufferBuilder

[LuciferYang]

What changes were proposed in this pull request?

Final follow-up in the Spark 3.2 cleanup series (#12525, #12541, #12550; open #12543 / #12548 / #12574).

The fallback path in ReadBufferBuilder.cpp at

if (!file_size.has_value())
{
    // only for spark3.2 file partition not contained file size
    // so first compute file size first
    auto tmp_read_buffer = std::make_unique<DB::ReadBufferFromHDFS>(...);
    file_size = tmp_read_buffer->getFileSize();
}

is still triggered on Spark 3.3+ whenever the Substrait LocalFilesNode was built without a fileSizes list — e.g. by IcebergLocalFilesBuilder — so the comment "only for spark3.2" is factually wrong on the currently supported version matrix.

This PR rewords the comment to describe the actual condition. The fallback code itself is unchanged.

How was this patch tested?

• Comment-only change — no code path touched.
• Verified via gluten-substrait/.../LocalFilesNode.java:194-203 that the fileProperties.setFileSize(...) block only fires when the caller passes a non-empty fileSizes list; producers such as IcebergLocalFilesBuilder do not pass one, so the fallback is unavoidable on Spark 3.3+.
• Repo-wide grep -rn 'spark3.2\|Spark 3.2' cpp-ch: 0 remaining occurrences after this change.
• clang-format was not run: the local toolchain is clang-format 12.0.1 and the project requires clang-format-15; running the mismatched version would produce a diff CI rejects (see AGENTS.md).

Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude claude-opus-4-7

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Pull request overview

This PR updates a misleading Spark 3.2-specific comment in the ClickHouse backend’s ReadBufferBuilder.cpp to better reflect when the file-size fallback path is taken (i.e., when Substrait LocalFiles inputs omit file size metadata), without changing runtime behavior.

Changes:

• Reworded the HDFS file-size fallback comment to be version-agnostic and describe the real triggering condition.

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Pull request overview

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