miner: enable more prefetch threads in local mining mode

[flywukong]

Description

1. Increase PrefetchMining parallelism for local mining when MEV is disabled. Keep MEV mode behavior unchanged .
2. Replaced return with continue in PrefetchMining to ensure the prefetch worker goroutine stays alive by skipping invalid transactions instead of terminating on error.

Rationale

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Example

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Changes

Notable changes:

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