Guard mempool drop percentage config

[giwaov]

Summary

• Normalize non-positive mempool drop percentages to the default value.
• Apply the same guard inside the internal drop helper.
• Add regression coverage for negative drop percentage eviction.

Why

A negative drop percentage could make mempool eviction calculate a zero or invalid drop count when transaction limits are reached. That can leave the eviction loop unable to make progress or cause invalid slicing.

Testing

• git diff --check
• Not run: Go toolchain is not installed in this environment.