fix: omnisharded resharding copy and replication

[levkk]

• fix: bubble up resharding COPY error; we were swallowing it previously
• fix: omnisharded table copies are distributed between shards evenly, avoiding duplicate copies of the same table data
• fix: omnisharded table replication uses upserts to 1) avoid unique key violation, 2) prevent race conditions between inserts into omni and sharded tables

[codecov]

Codecov Report

❌ Patch coverage is 30.66667% with 52 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...nd/replication/logical/publisher/publisher_impl.rs	0.00%	30 Missing ⚠️
...end/replication/logical/publisher/parallel_sync.rs	0.00%	15 Missing ⚠️
...src/backend/replication/logical/subscriber/copy.rs	0.00%	4 Missing ⚠️
...src/backend/replication/logical/publisher/table.rs	80.00%	3 Missing ⚠️

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