Safely normalize plugin query command/args joining for favorites

[multiplex55]

Motivation

• Ensure plugin search queries built from a saved favorite command plus optional args are tokenized and predictable by trimming and inserting a single space where appropriate.
• Make both programmatic resolution and the GUI favorites dialog use the same safe-join behavior so tokenized plugin commands work by default.

Description

• Add a reusable helper join_command_args(command: &str, args: Option<&str>) -> String in src/plugins/fav.rs that trims trailing whitespace from command, trims leading whitespace from args, and joins them with exactly one space only when args is present and non-empty.
• Update resolve_with_plugin to use join_command_args when building the query passed to plugin.search.
• Mirror the same safe-join logic in src/gui/fav_dialog.rs by importing and using join_command_args when expanding query: actions before calling plugin.search.
• Add unit tests in src/plugins/fav.rs covering: command="todo", args="list" -> todo list; empty/blank args leaves command unchanged; extra spacing is normalized; and resolve_with_plugin uses the normalized query.

Testing

• Added unit tests under the #[cfg(test)] module in src/plugins/fav.rs and attempted to run them with cargo test src/plugins/fav.rs -- --nocapture.
• Test run failed in this environment due to the installed rustc (1.87.0) being older than versions required by transitive crates (time requires 1.88.0), so the tests could not be executed here.

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Codex Task