chain/ethereum, graph: Patch missing sha3Uncles in pre-Gingerbread Celo blocks

[tmigone]

Overview

Since v0.42.0, every Celo block before the Gingerbread hard fork (block 21616000) fails to deserialize due to missing sha3Uncles from block responses, stalling subgraph indexing. See #6738 for more details. This PR fixes that issue. Now Celo transports re-parse block responses to set a default value for the missing sha3Uncles (other chains pay one eth_chainId call per provider).

Approach

• Zero hash, not the empty-uncles hash 0x1dcc…. The rust-web3 fork filled in zero. Mappings read the field as ethereum.Block.unclesHash, so any other value could change entity data compared with deployments indexed before v0.42.0.
• Detected by chain ID, not a provider feature flag. No config change is needed. The transport is built before the chain ID is known, which is why detection is a lazy eth_chainId call.