fix: prepared statements eviction CPU churn

[levkk]

When prepared statements were frequently evicted, the algorithm that was calculating the memory usage for the prepared statement data structure iterated over every single statement in the cache. This was effectively O(n^2) in the worst case, and it was burning CPU to provide only an approximation.

This change uses a simpler algorithm to calculate memory usage that doesn't have to walk the entire data structure.

[codecov]

Codecov Report

❌ Patch coverage is 86.84211% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/frontend/prepared_statements/mod.rs	89.28%	3 Missing ⚠️
pgdog/src/backend/prepared_statements.rs	80.00%	2 Missing ⚠️

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