[improvement](agg) Add passthrough local exchange before streaming ag…

[zclllyybb]

…gregation

Problem: non-final streaming aggregation can consume rows directly from its upstream pipeline, even when separating the pipelines with a local passthrough exchange is required to match the intended Repeat-to-streaming-aggregation execution shape. opt_perf_4.1 already carries the thrift field but does not expose or consume it end to end.

Solution: expose enable_local_exchange_before_agg as a default-enabled session variable and propagate it to TQueryOptions. StreamingAggOperatorX requests PASSTHROUGH only for non-final streaming aggregation. The existing hash-join passthrough priority and explicit shuffled_agg_ids HASH_SHUFFLE override remain intact; disabling the variable, or receiving an unset field from an older FE, retains the original distribution behavior.

Validation: ./build.sh --fe; ./build.sh --be -j 48; ./run-fe-ut.sh --run org.apache.doris.qe.VariableMgrTest; ./run-be-ut.sh --run --filter=StreamingAggOperatorTest.require_passthrough_local_exchange_before_non_finalize_agg -j 90. On an isolated FE/BE, an uncached GROUPING SETS query with agg_phase=2 produced REPEAT -> LOCAL_EXCHANGE(PASSTHROUGH) -> STREAMING_AGGREGATION by default; setting the variable false removed that local exchange while retaining the streaming aggregation and result rows.

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

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