fix(upower): resolve clippy warnings and remove dead code

[wayneColt]

Clean up the upower crate to pass cargo clippy with zero warnings.

• Replace fallible conversions (try_from) with infallible From impls where the conversion cannot fail
• Use idiomatic e.to_string() instead of format!("{}", e)
• Remove unused import and unnecessary .into() calls
• Remove commented-out code blocks and dead test module
• Fix copy-paste error in stream_battery_level log message

Verified: cargo clippy -p upower passes clean.

[wayneColt]

Happy to adjust if needed — just ran clippy on a few crates while reading through the shell architecture.