Add `hotkeypub` to bittensor-wallet

[basfroman]

For this task RaoFoundation/btcli#513 we need to have hotkeypub logic.

AC:
Wallet has methods and properties:

• hotkeypub (property)
• hotkeypub_file (property)
• get_hotkeypub
• set_hotkeypub
• unlock_hotkeypub
• regenerate_hotkeypub

[Nicknamess96]

I'd like to work on this. Planning to add hotkeypub functionality to bittensor-wallet, including public key export and display support.

[grnlemon]

You can go foward

[Nicknamess96]

Looking at the codebase, the core hotkeypub implementation is already in place — all 6 methods (hotkeypub, hotkeypub_file, get_hotkeypub, set_hotkeypub, unlock_hotkeypub, regenerate_hotkeypub) exist in both Rust and Python bindings.

I found a few gaps around the implementation that could use attention:

1. Inconsistent error mapping in Python bindings — regenerate_hotkeypub() and regenerate_coldkeypub() use a generic PyKeyFileError for all errors, while regenerate_coldkey() properly distinguishes PyValueError for invalid inputs vs PyKeyFileError for file errors
2. Missing edge case tests — no Python tests for invalid inputs to regenerate_hotkeypub(), no encryption/decryption test for hotkeypub files, and no lifecycle integration test

I'll put together a PR addressing these gaps.