server: Fix inverted htlc_recv_expiry bound on receive

Steven Rooserequested to merge
feat/floppy-vuln into master

Summary by CodeRabbit

  • Bug Fixes

    • Server now validates user-provided HTLC expiry for Lightning receive requests, rejecting values that don't leave sufficient safety margin relative to incoming HTLC expiries and chain height.

  • Tests

    • Added and adjusted tests covering HTLC expiry validation boundary conditions and accepted/rejected expiry cases.
Edited by coderabbit

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