Expand omni search tests to cover notes/todos, dedup, and ordering

[multiplex55]

Motivation

• Ensure the omni search plugin includes note and todo sources alongside apps/bookmarks/folders in integration scenarios.
• Prevent regressions from source expansion by verifying deduplication and deterministic ordering across merged sources.
• Preserve existing FST/dedup semantics and prior app/bookmark/folder behavior while adding new coverage.

Description

• Expanded tests/omni_search_plugin.rs to add a reusable setup_fixture() that uses tempdir() + set_current_dir(...) and sets ML_NOTES_DIR before seeding data.
• Seeded bookmarks, folders, notes and todos using official persistence helpers save_bookmarks, save_folders, save_notes, and save_todos in the fixture.
• Added integration-style tests: o_list_includes_notes_and_todos, o_list_with_query_filters_notes_todos_and_apps, o_prefix_matches_non_list_path, o_list_dedups_duplicate_rows_across_sources, and o_list_order_is_deterministic_for_same_input.
• Kept backward-compat coverage by preserving label_and_desc_same_returns_action and formatted the test file with cargo fmt.

Testing

• Ran cargo fmt -- tests/omni_search_plugin.rs, which completed successfully.
• Attempted cargo test --test omni_search_plugin, but the build failed in this environment due to the alsa-sys crate requiring a system alsa development package (alsa.pc) via pkg-config.
• Attempted cargo test --no-default-features --test omni_search_plugin, which also failed for the same missing system dependency.

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Codex Task