fix: insert splits + nextval

[levkk]

pgdog.nextval was not being rewritten for insert splits.

[codecov]

Codecov Report

❌ Patch coverage is 98.76033% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...frontend/router/parser/rewrite/statement/insert.rs	94.73%	2 Missing ⚠️
...c/frontend/router/parser/rewrite/statement/plan.rs	94.44%	1 Missing ⚠️

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