fix(chainstore): key fee history cache by request parameters

[zjuzhongwen]

Description

PassthroughStore.GetFeeHistory currently stores a single cached eth_feeHistory result without recording the parameters that produced it.

However, all three request parameters affect the response:

• blockCount
• newestBlock
• rewardPercentiles

As a result, a request made within the cache TTL can receive a result produced
for a different set of parameters.

For example:

GetFeeHistory(ctx, 1, "latest", nil)
GetFeeHistory(ctx, 100, "0x1234", []float64{10, 50, 90})

If the first result is still cached, the second call returns that result without
sending another RPC request. The returned fee history therefore does not
correspond to the second request.

This change:

• keys fee history cache entries by blockCount, newestBlock, and rewardPercentiles;
• uses a deterministic representation of rewardPercentiles;
• preserves the distinction between nil and an empty percentile list;
• removes expired entries or otherwise bounds the parameterized cache;
• adds regression tests for cache hits, parameter isolation, and TTL expiry.

There are no breaking API changes.

Jira / Linear Tickets

• DVT-000

Testing

• Test A
• Test B