wallet: disallow deriving ed25519 keys from default HD path

[tzemanovic]

Describe your changes

Follow-up to #4252. We disallow deriving ed25519 keys from the default HD path that's used for deriving shielded keys with the newly default modified ZIP32 to prevent accidental leak of these keys.

Checklist before merging

• If this PR has some consensus breaking changes, I added the corresponding breaking:: labels
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• If this PR requires changes to the docs or specs, a corresponding PR is opened in the namada-docs repo
  • Relevant PR if applies:
• If this PR affects services such as namada-indexer or namada-masp-indexer, a corresponding PR is opened in that repo
  • Relevant PR if applies:

[murisi]

Thanks! This will work. Just wanted to note two things that we have discussed already:

• Users who derive a private key using namadaw gen --alias ..., note the displayed mnemonic, and use this mnemonic to derive a shielded spending/viewing key will end up in the undesirable state where both the seed and the derived shielded spending/viewing key are both stored in the software wallet. This situation is worse if the user did key generation using the flag --unsafe-dont-encrypt, since that would mean that the seed is stored unencrypted.
• The functions Wallet::derive_store_hd_secret_key and Wallet::derive_store_key_from_mnemonic_code allow for the combined derivation and (potentially unencrypted) storage of modified ZIP 32 seed. So future changes to the codebase that make use of these functions might inadvertently bypass the checks made in this PR.

So while this PR blocks certain ways of reaching a state where a (potentially unencrypted) seed and its derivative shielded spending/viewing key are stored in the software wallet, there remain straightforward ways to enter this state from both the CLI and the internal APIs. And probably this situation is unavoidable due to the dual use of m/44'/877'/0'/0'/0' in namadaw gen's Ed25519 flow and also in the hardware wallet's modified ZIP 32 flow.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.02%. Comparing base (80223b3) to head (260ad01).
Report is 13 commits behind head on main.

Additional details and impacted files

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##             main    #4272   +/-   ##
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  Coverage   74.02%   74.02%           
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  Files         345      345           
  Lines      109920   109920           
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+ Hits        81367    81371    +4     
+ Misses      28553    28549    -4     

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