Fix aggregation flow with remote initiator

[ianton-ru]

Changelog category (leave one):

• Bug Fix (user-visible misbehavior in an official stable release)

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Fix aggregation flow with remote initiator

Documentation entry for user-facing changes

With object_storage_remote_initiator but without object_storage_cluster setting StorageObjectStorageCluster::getQueryProcessingStage returned QueryProcessingStage::Enum::FetchColumns, as result nodes sent all rows on initiator and aggregation executed on initiator.
Now method returns QueryProcessingStage::Enum::WithMergeableState is proper cases, and pre-aggregation executed on nodes.

CI/CD Options

Exclude tests:

• Fast test
• Integration Tests
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• Stateful tests
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• All with ASAN
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Regression jobs to run:

• Fast suites (mostly <1h)
• Aggregate Functions (2h)
• Alter (1.5h)
• Benchmark (30m)
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[ianton-ru]

@codex review

[github-actions]

Workflow [PR], commit [e72ebfe]

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep unsupported storages at FetchColumns

When object_storage_remote_initiator is set for a configuration that does not support cluster mode, such as icebergLocal or other local data-lake storages where getClusterName is empty because isClusterSupported is false, this new exception makes getQueryProcessingStage report WithMergeableState. IStorageCluster::read still immediately falls back to pure_storage for those configurations, and StorageObjectStorage::read ignores processed_stage, so aggregate queries can be planned as if partial aggregation happened remotely even though only raw rows were read. Please only return the distributed stage when the subsequent read path will actually use the remote/cluster execution path.

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