Allow non matching schemas on EXPORT PARTITION

[k-morozov]

Closes: #1717

Problem. EXPORT PART/EXPORT PARTITION matched source and destination columns positionally, exactly like INSERT INTO dest SELECT * FROM src, and required the column count to match exactly. This is unnecessarily strict for a common
real scenario: a MergeTree table's schema grows over time (new trailing columns added), but older partitions still need to be exported to an external table (Iceberg/object storage) whose schema was fixed when it was created. Such exports were rejected outright with NUMBER_OF_COLUMNS_DOESNT_MATCH, with no way to just drop the newer columns and export the rest.

Solution. Added the export_merge_tree_part_schema_mismatch_mode setting (default strict, preserving the old behavior). The new ignore_extra_source_columns_by_position mode allows the source to have more columns than the destination: the extra trailing source columns (by declared/readable position, the same order INSERT SELECT uses) are dropped, both at synchronous validation time (verifyExportSchemaCastable, so a real mismatch still fails immediately at ALTER TABLE ... EXPORT time) and at actual data-export time (ExportPartTask::addExportConvertingActions, via a projection step before the existing positional CAST). The reverse direction — destination has more columns than source — is still always rejected in both modes.

Known follow-up. Ignored columns are still fully read and decompressed from disk before being discarded in the projection step, so no I/O is saved. Skipping the read up front isn't safe as a naive truncation: a kept
ALIAS/MATERIALIZED column can legally forward-reference an ignored one, so it would need a real dependency-graph walk instead. Left as a possible future optimization

Changelog category (leave one):

• Improvement

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

Added the export_merge_tree_part_schema_mismatch_mode setting. With ignore_extra_source_columns_by_position (default: strict), EXPORT PART/EXPORT PARTITION allows a source table with extra trailing columns — they are simply
ignored instead of failing with NUMBER_OF_COLUMNS_DOESNT_MATCH.

Documentation entry for user-facing changes

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[github-actions]

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