rpc/jsonrpc: adhere to latest eth_config spec

[taratorio]

closes https://github.com/erigontech/security/issues/54

fixes:

• use current block time instead of wall time
• do not return err prior to Shanghai (instead support all fork ranges)
• handle case for forks activated at genesis (activationTime=0 when node is at genesis block) - reported by steel team at https://discord.com/channels/595666850260713488/753271902520213625/1424684089893720095
• return blobSchedule: null for forks which do not have it (prior to Cancun)

[taratorio]

reminder to self: cherry-pick to release/3.2 after merge

[Copilot]

Pull Request Overview

This PR updates the eth_config RPC endpoint implementation to comply with the latest specification. The changes address several edge cases and specification requirements around fork configuration and blob schedules.

Key changes:

• Uses current block time instead of wall clock time for fork determination
• Returns blobSchedule: null for pre-Cancun forks instead of erroring
• Handles genesis block edge case where activation time should be 0

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

File	Description
rpc/jsonrpc/eth_system.go	Updated Config method to use block time, removed pre-Cancun error, handle genesis case, changed BlobSchedule to nullable
rpc/jsonrpc/eth_system_test.go	Added test cases for genesis and pre-Cancun scenarios, updated test structure to support head state
execution/chain/chain_config.go	Changed GetBlobConfig to return nil for pre-Cancun, added nil checks in dependent methods
rpc/jsonrpc/testdata/eth_config/*.json	Added test fixtures for new scenarios

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