fix: guard against empty baseFeePerGas in FeeHistoryEstimator.RefreshDynamicPrice

[snowkide]

Summary

FeeHistoryEstimator.RefreshDynamicPrice indexes into feeHistory.BaseFee with
feeHistory.BaseFee[len(feeHistory.BaseFee)-1] without checking the slice is
non-empty. If an eth_feeHistory RPC call returns err == nil with an empty
baseFeePerGas array — which we've observed real providers do instead of
returning an error, e.g. on a deep/archive block range they can't serve, or on
chains without full EIP-1559 support — this indexes with -1 and panics:

panic: runtime error: index out of range [-1]

goroutine ... [running]:
github.com/smartcontractkit/chainlink-evm/pkg/gas.(*FeeHistoryEstimator).RefreshDynamicPrice(...)
	pkg/gas/fee_history_estimator.go:243 +0x8e5
github.com/smartcontractkit/chainlink-evm/pkg/gas.(*FeeHistoryEstimator).run(...)
	pkg/gas/fee_history_estimator.go:130 +0x105

Because RefreshDynamicPrice runs in an unrecovered background goroutine
(FeeHistoryEstimator.run), this panic is fatal to the entire node process —
not just the affected chain's gas estimator. Every chain and job the node
services goes down until the process is restarted.

Note the Reward slice a few lines below this is already defended
(if len(reward) < 2 { continue }); BaseFee was not.

Impact observed

We hit this in production across three separate CCIP node deployments running
different chainlink-evm builds (pseudo-versions from Feb, Mar, and Jun 2026),
on different physical hosts, recurring in bursts over several weeks before we
root-caused it. Log history shows it's been reachable since at least the
February 2026 build.

Fix

Added a bounds check right after the existing RPC-error check, returning a
descriptive error instead of indexing into the empty slice — the same
treatment already given to a malformed Reward response a few lines down.

Testing

Added TestRefreshDynamicPrice_EmptyBaseFee, which reproduces the panic
against the real package using its own mocks (feeding an empty BaseFee with
err == nil) and confirms the fix returns a graceful error instead. Full
existing pkg/gas test suite passes unmodified; go vet is clean.