validate --data-dir is not a file before startup

[evanbranigan-dotcom]

Fixes: #2272

When --data-dir points to a wallet file (e.g. --data-dir /path/to/zingo-wallet.dat) instead of its parent directory, zingolib panics with:

panic!("Couldn't create zcash directory!")

This gives no indication that the path should be a directory, not a file.

Changes

• Add early validation in ConfigTemplate::fill() that checks if the provided --data-dir path is an existing file
• Return a clear error message with the correct parent directory as a hint:

  --data-dir must be a directory, not a file.
  You provided: /path/to/zingo-wallet.dat
  Hint: use the parent directory instead, e.g. --data-dir /path/to
  

• Add test data_dir_is_a_file in the error_cases module
• All 71 existing tests pass

AI disclosure: Fix implemented with Claude Code (Anthropic), reviewed and verified by human contributor.

[Oscar-Pepper]

great work! the formatting seems incorrect during testing, the "\n" newline characters are not working. could we fix this?

example:
$ ./target/release/zingo-cli --data-dir ~/wallets/test_5/zingo-wallet.dat
Error filling config template: "--data-dir must be a directory, not a file.\nYou provided: /home/oscar/wallets/test_5/zingo-wallet.dat\nHint: use the parent directory instead, e.g. --data-dir /home/oscar/wallets/test_5"

[evanbranigan-dotcom]

Thanks for catching this! The newlines in the error string are correct — the issue is in run_cli() at line 623, which uses {e:?} (Debug formatting) to print the error. Debug wraps the string in quotes and escapes \n as literal characters.

Changing {e:?} to {e} fixes it — and would also fix the existing server URI error on line 431 which has the same \n pattern.

Since that's pre-existing code outside the scope of this PR, would you prefer I fix it here as a second commit, or should I open a separate issue for it?