fix: resolve clippy warnings, broken doctest, and add unit tests

[Nicknamess96]

Summary

Fix compiler warnings, a broken doctest, and add comprehensive unit tests to the bittensor-wallet Rust library.

Changes

• Fix 5 mismatched_lifetime_syntaxes warnings in PyO3 binding functions by adding explicit '_ lifetime annotations to Cow<[u8]> return types
• Fix broken doctest in Wallet::create that caused cargo test --doc to fail by converting Python-style docstring to idiomatic Rust /// # Arguments format
• Apply cargo fmt to resolve pre-existing formatting inconsistencies in python_bindings.rs
• Add 66 unit tests across 5 modules (config, keypair, keyfile, wallet, utils), increasing test coverage from 1 to 67 total tests

Testing

• cargo test --lib passes (67/67 tests)
• cargo test --doc passes (0 failures, previously 1 failure)
• cargo fmt --check clean
• cargo build produces 0 lifetime warnings (previously 5)

[basfroman]

pls resolve the conflicts after rebasing to staging

[Nicknamess96]

Rebased onto staging and resolved conflicts — ready for re-review.

[Nicknamess96]

Hey @basfroman — just a heads-up on the cargo test failure: the error is a pyo3 linker issue (undefined symbol: Py_InitializeEx, _Py_NoneStruct, etc.), not related to the changes in this PR.

The check-rust.yml workflow runs cargo test --workspace --all-features, which enables the pyo3 feature and requires Python development headers to link. But the CI job doesn't install python3-dev, so the linker can't resolve the Python symbols.

This appears to have been introduced when the workflow was simplified in #177 (the old cargo-check-no-python job was removed).

The newer CI runs after my rebase are also pending your approval (action_required for first-time contributors). Could you approve those when you get a chance? Thanks!

[Nicknamess96]

Just to confirm — I checked the latest Check Rust CI run on the staging branch itself (run #22327290507), and it also fails cargo test with the same pyo3 linker errors. In fact, staging has more failures than this PR (clippy, fmt, and zepter also fail there).

This PR actually fixes the clippy and doctest issues — the only remaining failure (cargo test) is inherited from staging's broken CI config.

Happy to help with a fix for the CI workflow too if that would be useful!

[Nicknamess96]

Hey @basfroman — rebased onto staging and all conflicts are resolved. The cargo test failure is a pre-existing pyo3 linker issue that also fails on staging itself. Ready for re-review whenever you get a chance.