Gh 685 review2

[czarte]

[!NOTE] Clarifies blockchain service URL help text and error messaging, fixes a misattributed configurator error, and updates tests to no longer pass --blockchain-service-url where unnecessary.

• CLI/Help:
  • Update BLOCKCHAIN_SERVICE_HELP with <api-key> placeholders, improved grammar, and explicit --chain base-mainnet guidance.
• Error handling:
  • Change required-arg message to singular phrasing; fix into_configurator_error target from gas-price to blockchain-service-url.
• Unit/Integration Tests:
  • Adjust expectations for updated help/error strings.
  • Add helper to validate exceptions suppressing blockchain URL requirement.
  • Remove --blockchain-service-url from many MASQNode::start_* calls; minor arg tweaks (e.g., chain identifier from DEFAULT_CHAIN).

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