Dash InstantSend: set min fee default to 1 sat/vB and preserve 0 as gate-off

Closed in favor of !772 (merged)

Summary

Adjust Dash InstantSend fee-rate defaulting so configuration is more practical and ensure the "0 disables gate" behavior.

Changes

• Change defaultDashInstantSendMinFeeRateSatPerVByte from 8 to 1.0.
• Update DashInstantSendConfiguration.WithDefaults() to apply the default only when value is < 0 (not <= 0).

Why

• 8 sat/vB is overly strict for Dash and can reject normal wallet transactions.
• 0 should remain a valid explicit value to disable the fee-rate gate.

Summary by CodeRabbit

• Bug Fixes
  • Dash InstantSend min-fee validation now preserves 0 as "disabled"; only negative values revert to the default.
• Configuration
  • DASH InstantSend default minimum fee updated to 1.5 sat/vB (was 0).
• Behavior
  • Simulation lowers low-fee cap to allow more low-fee paths for Dash InstantSend.
  • Improved mempool fee-rate handling for better InstantSend fee assessment.
• Tests
  • Added tests to verify zero-preservation and fee-rate calculations.