contrib: avoid echoing supplied RPC passwords #36433

Motivation

Printing a user-supplied password again exposes a secret without any benefit. This is especially unnecessary in getpass mode: the password is entered hidden, then printed to the screen.

Changes

Show the password only when rpcauth.py generated it. Do not echo positional or prompted passwords. JSON output and rpcauth generation remain unchanged; CLI output tests were added.

Testing

• python3 -m py_compile share/rpcauth/rpcauth.py
• python3 -m py_compile test/functional/tool_rpcauth.py
• ./ci/lint.py --lint=py_lint
• tool_rpcauth.py in an Ubuntu 24.04 container
• Manual checks for:
  • generated password output
  • positional password input
  • prompted password input (- / getpass)
  • --json output

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External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36433
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36433.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36433#author-ralph

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Reviews

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To preserve the time of reviewers, this project requires authors to understand the code they are submitting. Given that the description is entirely LLM generated, I don't think that is the case, so I am closing this again. The change also doesn't seem too useful to me, given that this is a script specifically to create credentials, and not to transport or verify them.