Return dialog for exact todo query and preserve submenu for todo

[multiplex55]

Motivation

• Make the root todo query show the primary dialog action instead of listing submenu commands when the user types exactly todo (case-insensitive) with no trailing text.
• Preserve discoverability of submenu commands when the user types todo (with a trailing space) or todo <command>.

Description

• Update TodoPlugin::search_internal to detect an exact todo match and return a single Action with action == "todo:dialog" and label "todo: edit todos" when the suffix is exactly empty.
• Keep the existing submenu action list in the todo branch when the suffix contains only whitespace so todo still shows all subcommands.
• Change the entry-point trimming in Plugin::search to use trim_start() so trailing spaces are preserved and the plugin can distinguish "todo" from "todo ".
• Add a unit test search_todo_space_shows_submenu and simplify search_plain_todo_opens_dialog in tests/todo_plugin.rs to assert exact behaviors for "todo" and "todo " respectively.

Testing

• Added unit tests in tests/todo_plugin.rs verifying that plugin.search("todo") returns exactly one result with action == "todo:dialog" and that plugin.search("todo ") exposes the submenu labels; the tests were added successfully.
• Ran cargo test --test todo_plugin, but the build failed during dependency compilation due to a missing system library required by alsa-sys (error: missing alsa.pc / alsa development package), so the test run could not complete in this environment.

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Codex Task