feat: block cross-shard sharding key updates with ON DELETE

[murex971]

Description

Block cross-shard sharding key updates when the row is referenced by a foreign key with a destructive ON DELETE action (CASCADE, SET NULL, or SET DEFAULT). Those moves are implemented as DELETE + INSERT, so the delete step could silently affect related rows. PgDog now checks loaded schema metadata before the delete/insert and returns a recoverable error; same-shard updates and ON DELETE RESTRICT / NO ACTION are unchanged.

Testing

cargo nextest run --profile dev test_foreign_key_on_delete_sharding_key_update

 cargo nextest run --profile dev 'frontend::router::parser::rewrite::statement::update::test'

 cargo nextest run --profile dev 'frontend::client::query_engine::multi_step::test'

Related issues/comments

closes #899

[levkk]

Nice!

[codecov]

Codecov Report

❌ Patch coverage is 96.31579% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../frontend/client/query_engine/multi_step/update.rs	85.71%	3 Missing ⚠️
...frontend/router/parser/rewrite/statement/update.rs	92.85%	3 Missing ⚠️
...tend/client/query_engine/multi_step/test/update.rs	99.05%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[levkk]

I feel like this logic is duplicated, but I could be wrong.