fix: use SLOT_DURATION_MS instead of deprecated SECONDS_PER_SLOT

[barnabasbusa]

Fix:

panic: runtime error: integer divide by zero
  
  goroutine 178 [running]:
  github.com/ethpandaops/ethwallclock.(*DefaultSlotCreator).FromTime(0xc0015b4b00, {0x0?, 0x0?, 0x2e54ba0?})
  	/home/runner/go/pkg/mod/github.com/ethpandaops/ethwallclock@v0.4.0/slot_creator.go:28 +0x70
  github.com/ethpandaops/ethwallclock.(*DefaultSlotCreator).Current(0xc0015b4b00)
  	/home/runner/go/pkg/mod/github.com/ethpandaops/ethwallclock@v0.4.0/slot_creator.go:24 +0x2b
  github.com/ethpandaops/ethwallclock.NewEthereumBeaconChain.func1()
  	/home/runner/go/pkg/mod/github.com/ethpandaops/ethwallclock@v0.4.0/beacon_chain.go:44 +0x86
  created by github.com/ethpandaops/ethwallclock.NewEthereumBeaconChain in goroutine 47
  	/home/runner/go/pkg/mod/github.com/ethpandaops/ethwallclock@v0.4.0/beacon_chain.go:36 +0x207
  

Summary

• Replace SecondsPerSlot (time.Duration, yaml:"SECONDS_PER_SLOT") with SlotDurationMs (uint64, yaml:"SLOT_DURATION_MS") in ChainSpec to match the new consensus spec field
• Fall back to SECONDS_PER_SLOT for backwards compatibility with older genesis configs that haven't migrated yet
• Add a guard in InitWallclock to prevent divide-by-zero panic if neither value is available

Context

SECONDS_PER_SLOT has been deprecated and removed from the consensus spec config (ethereum/consensus-specs#4926) in favor of SLOT_DURATION_MS. The ethereum-genesis-generator is removing it in ethpandaops/ethereum-genesis-generator#268. When CL nodes stop returning SECONDS_PER_SLOT, the ChainSpec field stays at zero, causing a divide-by-zero panic in ethwallclock when computing the current slot.

Similar change was made in dora: ethpandaops/dora#612

Test plan

• Verify assertoor starts correctly with genesis configs that include SLOT_DURATION_MS but not SECONDS_PER_SLOT
• Verify backwards compat with configs that still include SECONDS_PER_SLOT

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