fix(clippy): resolve pedantic and nursery warnings from Rust 1.95

[JanZachmann]

Address warnings surfaced by clippy under Rust 1.95:

• collapsible match guards in reconnection.rs
• sort_by_key with Reverse instead of sort_by closure
• redundant clones in tests and device service client
• String::new() instead of "".to_string()
• items_after_statements refactors
• pass-by-ref for generate_token; byte-literal and inline format args
• u8::try_from for SESSION_KEY_LEN cast
• implicit_clone fixes in network ini writing
• unused headers collection removed in http_client test

Annotate intentional exceptions:

• #[allow(clippy::future_not_send)] on actix test helpers (actix uses Rc internally, so the future cannot be Send)
• #[allow(clippy::await_holding_lock)] on firmware tests that intentionally serialize access to the data folder via std::sync::Mutex