fix: omnishard fanout should be opaque

[levkk]

Don't return INSERT <number of shards>, UPDATE <same> for omnisharded writes. The client needs to believe it's just one table.

[codecov]

Codecov Report

❌ Patch coverage is 96.07843% with 24 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../frontend/client/query_engine/start_transaction.rs	25.00%	9 Missing ⚠️
pgdog/src/frontend/client/query_engine/discard.rs	0.00%	3 Missing ⚠️
pgdog/src/frontend/client/query_engine/pub_sub.rs	0.00%	3 Missing ⚠️
...gdog/src/frontend/client/query_engine/test/omni.rs	98.43%	3 Missing ⚠️
...g/src/frontend/client/query_engine/test/sharded.rs	98.19%	3 Missing ⚠️
pgdog/src/net/messages/mod.rs	72.72%	3 Missing ⚠️

📢 Thoughts on this report? Let us know!