`estimate_route_fee` failed with unexpected CLTV delta

[yyforyongyu]

Found from this CI, which doesn't look like a flake,

--- FAIL: TestLightningNetworkDaemon (535.14s)
    harness_setup.go:37: Setting up HarnessTest...
    harness_setup.go:50: Prepare the miner and mine blocks to activate segwit...
    harness_setup.go:88: Using miner backend=btcd
    harness_setup.go:58: Connecting the miner at 127.0.0.1:10019 with the chain backend...
    --- FAIL: TestLightningNetworkDaemon/tranche13/279-of-324/bitcoind/estimate_route_fee (75.66s)
        harness_node.go:395: Starting node (name=Alice) with PID=33185
        harness_node.go:395: Starting node (name=Bob) with PID=33247
        harness_node.go:395: Starting node (name=carol) with PID=33334
        harness_node.go:395: Starting node (name=dave) with PID=33397
        harness_node.go:395: Starting node (name=eve) with PID=33430
        harness_node.go:395: Starting node (name=Paula) with PID=34031
        harness_node.go:395: Starting node (name=Frank) with PID=34050
        harness_node.go:395: Starting node (name=ImWithoutChannels) with PID=34434
        lnd_estimate_route_fee_test.go:495: 
            	Error Trace:	/home/runner/work/lnd/lnd/itest/lnd_estimate_route_fee_test.go:495
            	            				/home/runner/work/lnd/lnd/itest/lnd_estimate_route_fee_test.go:449
            	Error:      	Not equal: 
            	            	expected: 1076
            	            	actual  : 976
            	Test:       	TestLightningNetworkDaemon/tranche13/279-of-324/bitcoind/estimate_route_fee
            	Messages:   	cltv delta
        --- FAIL: TestLightningNetworkDaemon/tranche13/279-of-324/bitcoind/estimate_route_fee/probe_based_estimate,_multiple_different_public_LSPs (0.70s)
            testing.go:1811: test executed panic(nil) or runtime.Goexit: subtest may have called FailNow on a parent test
        harness.go:398: finished test: estimate_route_fee, start height=700, end height=713, mined blocks=13
        harness.go:357: test failed, skipped cleanup
    lnd_test.go:152: Failure time: 2026-03-26 11:37:19.117
FAIL