Fix fee estimation bounds

[a-shannon]

Closes #1884.

Summary

• Floor recommended fees at the configured node minimal fee so /transactions/getFee cannot recommend a fee below the accepted minimum.
• Cap expected wait-time calculations to the mempool statistics window so long node idle uptime does not dominate the estimate.
• Keep HTTP parameter validation separate from Harden REST API input validation #2374; this PR fixes the core mempool estimation behavior.

Tests

• sbt "testOnly org.ergoplatform.nodeView.mempool.ErgoMemPoolSpec"
• sbt "testOnly org.ergoplatform.http.routes.TransactionApiRouteSpec"
• publication guard