fix: detect and handle pg_dump end of input COPY marker

[levkk]

This is generated by pg_dump when copying tables around and piped to psql directly. This is common in migration tooling.

[codecov]

Codecov Report

❌ Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/frontend/router/parser/copy.rs	95.83%	1 Missing ⚠️

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