Tweak word completions more (#37697)

Follow-up of https://github.com/zed-industries/zed/pull/37352
Closes https://github.com/zed-industries/zed/issues/37132

* disabled word completions in the agent panel's editor
* if not disabled, allow to trigger word completions with an action even
if the completions threshold is not reached

Release Notes:

- Fixed word completions appearing in the agent panel's editor and not
appearing when triggered with the action before the completion threshold
is reached

crates/editor/src/code_context_menus.rs

@@ -251,7 +251,7 @@ enum MarkdownCacheKey {
 pub enum CompletionsMenuSource {
     Normal,
     SnippetChoices,
-    Words,
+    Words { ignore_threshold: bool },
 }
 
 // TODO: There should really be a wrapper around fuzzy match tasks that does this.