perf: borrowed slice adjusted for empty ssid check

[cachebag]

Change decode_ssid_or_empty to return a borrowed slice instead of String

[Copilot]

Pull request overview

This PR optimizes the decode_ssid_or_empty function by changing its return type from String to Cow<'static, str> to reduce allocations by returning borrowed slices when possible. The function is used for SSID comparison throughout the codebase, and avoiding allocations can improve performance.

• Changed decode_ssid_or_empty return type from String to Cow<'static, str>
• Updated call sites to convert Cow to String where needed using .to_string()

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
nmrs/src/util/utils.rs	Modified decode_ssid_or_empty to return Cow<'static, str> instead of String, returning Cow::Borrowed("") for empty/invalid cases and Cow::Owned(s.to_owned()) for valid UTF-8
nmrs/src/monitoring/info.rs	Updated two call sites to convert the Cow return value to String using .to_string() where String is required by the function signature

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