feat(editor): clear results when clearing a query, add Clear Results menu

[datlechin]

Closes #1256.

The trash "Clear Query" button cleared the editor text but left the previous query's results in the panel, with no way to remove them.

Changes

• The trash button now clears the query and its results together.
• A new "Clear Results" item on the results right-click menu clears results while keeping the query, shown for query tabs that have results.
• Both route through a shared clearActiveQueryResults() that reuses the existing result-reset path. No confirmation, since re-running the query repopulates the panel.

Tests

ClearQueryResultsTests covers the clear path, query-text preservation, and the query-tab gating.