Function remove_contexts_for_block 

pub fn remove_contexts_for_block(block_id: &str)

Remove every BROWSER_PANE_HWND_CONTEXT entry whose context refers to the given block_id. Called from on_before_close_browser_pane so the map doesn’t grow unbounded as panes are opened and closed over the session. Keyed by block_id (not HWND) because the close path has the label/block_id immediately but not the HWND — by the time CEF fires on_before_close, the browser’s HWND may already be invalid.