fix: race in extended_anonymous prepared statements

[levkk]

• fix: anonymous prepared statements were self-evicting immediately upon insertion because they are all named "".
• refactor: server connections were getting the prepared statement and pooler mode update on fast path only; this didn't affect anything materially, since the fast path was executed first, but created confusion

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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