Avoid iterator allocation for prepared ExecuteNonQuery

[skuirrels]

Stacked on Giorgi#340 so the diff stays focused on this follow-up.

Problem

Prepared ExecuteNonQuery() still goes through an IEnumerable iterator even though a reusable prepared command contains one statement. That adds a fixed 112-byte allocation to every execution.

Solution

Execute that single prepared result directly, while keeping the existing iterator path for readers, unprepared commands, and multi-statement commands. Prepared-statement lifecycle and deferred disposal still use the same shared bookkeeping.

Benchmark

Median of 7 runs after warmup, 20,000 executions per run:

Workload	Before	After	Change
No parameters	112 B/op	0 B/op	-100%
Three parameters	232 B/op	120 B/op	-48.3%

The remaining 120 bytes come from parameter-value wrappers and are outside this change. Timing varied within noise; the allocation reduction was stable.

Tests

• DuckDBCommandTests: 13 passed
• Full suite: 6,907 passed, 0 failed

[skuirrels]

This is stacked on Giorgi#340 and should only be merged after Giorgi#340 has been merged. I have therefore left in draft state.