Mark BufferExec and AnalyzeExec as eager

[geoffreyclaude]

Which issue does this PR close?

• Closes BufferExec and AnalyzeExec should report eager evaluation #22708.

Rationale for this change

BufferExec and AnalyzeExec both have eager evaluation behavior: they can drive child streams in spawned tasks instead of performing exactly one downstream poll worth of work at a time. Their PlanProperties currently report EvaluationType::Lazy, so physical-plan metadata does not describe how these operators execute.

While making that metadata accurate, this PR also keeps need_data_exchange(...) scoped to its original purpose. #4585 introduced that helper as a way to identify physical operators that require exchange-style handling because they redistribute partitions or gather multiple input partitions into one output partition. #4586 implemented it for the native exchange/gather operators called out there: non-round-robin RepartitionExec, CoalescePartitionsExec, and SortPreservingMergeExec.

The later cooperative-scheduling work in #16398 introduced EvaluationType and made need_data_exchange(...) use evaluation_type == EvaluationType::Eager. That shortcut worked when the eager operators and exchange/gather operators were the same set. With BufferExec and AnalyzeExec correctly classified as eager, the shortcut would make need_data_exchange(...) report operators that do eager child polling but do not perform a data exchange.

So the intended split in this PR is:

• EvaluationType describes execution behavior.
• need_data_exchange(...) identifies partition redistribution or partition gathering.

What changes are included in this PR?

• Mark BufferExec as EvaluationType::Eager while keeping its existing cooperative scheduling metadata.
• Mark AnalyzeExec as EvaluationType::Eager in its computed plan properties.
• Clarify the EvaluationType docs so eager evaluation is not defined by whether work starts in execute or on the first stream poll.
• Restore need_data_exchange(...) as an exchange/gather predicate for the native exchange operators instead of deriving it from all eager operators.

Are these changes tested?

Targeted tests pass:

cargo test -p datafusion-physical-plan --lib buffer::tests
cargo test -p datafusion-physical-plan --lib analyze::tests
cargo test -p datafusion-physical-plan --doc execution_plan
cargo test -p datafusion-physical-plan --lib execution_plan::tests::buffer_exec_does_not_need_data_exchange
RUSTDOCFLAGS="-D warnings" cargo doc -p datafusion-physical-plan --no-deps

I also added a focused regression test that BufferExec does not require data exchange. I did not add tests that merely assert the assigned EvaluationType, as those would duplicate the implementation rather than cover behavior.

Are there any user-facing changes?

No query-result changes are expected. This updates physical-plan metadata and keeps need_data_exchange(...) scoped to operators that move data between partitions or gather partitions together.

[gabotechs]

👍 +1 from me, this looks more correct now.

[gabotechs]

Some operators that are correctly marked as Eager, but that this function was wrongly treating them as need_data_exchange() -> true:

• DataSinkExec
• RepartitionExec when Partitioning::RoundRobin

[geoffreyclaude]

need_data_exchange was pretty broken then... As discussed privately, we should look into deprecating then deleting this method, as probably no-one is using it (at least correctly.)

[gabotechs]

I'll leave this here marinating for a day so that other people can chime in, otherwise, I'll merge it tomorrow