fix: handle expressions in shard key UPDATE statements

[levkk]

• fix: handle expressions in UPDATE statements that change the sharding key
• chore: clippy

UPDATE users SET id = $1, created_at = now() WHERE id = $2

[codecov]

Codecov Report

❌ Patch coverage is 95.58011% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...frontend/router/parser/rewrite/statement/update.rs	92.94%	6 Missing ⚠️
...tend/client/query_engine/multi_step/test/update.rs	98.80%	1 Missing ⚠️
.../frontend/client/query_engine/multi_step/update.rs	88.88%	1 Missing ⚠️

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