feat: chatPrefillFromUser hook handler — substitute @ai for AI's display name

Override the default "@<name> " chat-input prefill with the configured
AI trigger ("@ai " by default) when a user clicks the AI's row in the
user list. Without this, clicking the AI chip would prefill
"@AI_Assistant ", which doesn't match anything the server-side mention
extractor recognises.

What this PR adds:
- clientVars server hook exposing { trigger, authorName, authorId } at
  clientVars.ep_ai_chat so the client can recognise the AI's row.
- chatPrefillFromUser client hook that returns the trigger string when
  the clicked authorId matches the AI's; otherwise returns nothing so
  core's default ("@<name> ") wins.
- Mocha unit test (static/tests/backend/specs/chat_prefill.ts) covering
  AI/non-AI/missing-clientVars/custom-trigger/missing-trigger paths.

Depends on ether/etherpad#7660 which adds the chatPrefillFromUser hook
to core. On older cores the hook is never fired and this PR is a
benign no-op — graceful degradation, no install requirement bump.

Originally this PR shipped the entire user-list click handler inside
the plugin. Per review feedback, the generic "click a user → prefill
@-mention" UX belongs in core (it's a discoverability win for any
multi-user pad, AI or no AI), so the bulk moved to ether/etherpad#7660
and this PR is now ~40 lines of glue.

(Replaces the previous 3-commit history on this branch with a clean
rebase against the new main, which absorbed PRs #18 and #20 since the
branch was opened.)

Author: JohnMcLear

ep.json

@@ -6,11 +6,13 @@
         "loadSettings": "ep_ai_chat/index",
         "handleMessage": "ep_ai_chat/index",
         "userJoin": "ep_ai_chat/index",
-        "socketio": "ep_ai_chat/index"
+        "socketio": "ep_ai_chat/index",
+        "clientVars": "ep_ai_chat/index"
       },
       "client_hooks": {
         "postAceInit": "ep_ai_chat/static/js/index",
-        "chatSendMessage": "ep_ai_chat/static/js/index"
+        "chatSendMessage": "ep_ai_chat/static/js/index",
+        "chatPrefillFromUser": "ep_ai_chat/static/js/index"
       }
     }
   ]

index.js

@@ -152,6 +152,23 @@ exports.socketio = (hookName, {io}) => {
 };
 exports.getSocketIo = () => socketioRef;
 
+/**
+ * clientVars: expose the trigger string and the AI author's identity to
+ * the client. The chatPrefillFromUser client hook uses this to recognise
+ * the AI's row in the user list and substitute "@ai " for the otherwise
+ * useless "@AI_Assistant " prefill.
+ */
+exports.clientVars = async (hookName, context) => {
+  const aiId = await getAiAuthorId();
+  return {
+    ep_ai_chat: {
+      trigger: chatSettings.trigger,
+      authorName: chatSettings.authorName,
+      authorId: aiId,
+    },
+  };
+};
+
 /**
  * userJoin: when a client joins a pad, push the AI's author info to the
  * room so the AI shows up in the editors list immediately — without

static/js/index.js

@@ -23,6 +23,27 @@ exports.postAceInit = (hookName, context) => {
   } catch (e) { /* never break ace init */ }
 };
 
+/**
+ * chatPrefillFromUser: when Etherpad core prefills "@<name> " in the chat
+ * input on a user-list click, swap in the configured trigger string for
+ * the AI's row. Without this, clicking the AI's chip in the user list
+ * would prefill "@AI_Assistant " which doesn't match anything the
+ * server-side mention extractor recognises.
+ *
+ * Requires the chatPrefillFromUser hook (Etherpad >= the version that
+ * landed ether/etherpad#7660). On older cores this hook is silently
+ * never called and clicks fall back to the default prefill.
+ */
+exports.chatPrefillFromUser = (hookName, context, cb) => {
+  try {
+    const ai = (window.clientVars && window.clientVars.ep_ai_chat) || {};
+    if (ai.authorId && context && context.authorId === ai.authorId) {
+      return cb(`${ai.trigger || '@ai'} `);
+    }
+  } catch (_e) { /* fall through to default */ }
+  return cb();
+};
+
 /**
  * chatSendMessage: before a chat message is sent, capture the current
  * selection in the pad and attach it to the message.

static/tests/backend/specs/chat_prefill.ts

@@ -0,0 +1,95 @@
+'use strict';
+
+import {strict as assert} from 'assert';
+
+/**
+ * The chatPrefillFromUser client hook — exercised here as a plain
+ * function (it's a CommonJS export from static/js/index.js) with a
+ * stubbed `window.clientVars`. Lets us cover the AI-vs-human branch
+ * without standing up a browser.
+ *
+ * The companion frontend test in Etherpad core (#7660) covers what
+ * the browser actually does with the returned string.
+ */
+
+const Module = require('module');
+
+// The static/js client bundle requires nothing Node-specific, so we
+// can require it directly. But it reads `window.clientVars`; install
+// a global shim before requiring.
+const installWindowShim = (clientVars: any) => {
+  // @ts-ignore
+  global.window = {clientVars};
+};
+
+const clearWindowShim = () => {
+  // @ts-ignore
+  delete global.window;
+  // Drop the client module from the require cache so a follow-up
+  // require() re-reads it under the new shim.
+  const path = require.resolve('../../../../static/js/index');
+  delete Module._cache[path];
+};
+
+describe('ep_ai_chat - chatPrefillFromUser hook handler', function () {
+  afterEach(() => clearWindowShim());
+
+  it('returns the configured trigger when the clicked user is the AI', function (done) {
+    installWindowShim({ep_ai_chat: {trigger: '@ai', authorId: 'a.ai_42'}});
+    const {chatPrefillFromUser} = require('../../../../static/js/index');
+    chatPrefillFromUser('chatPrefillFromUser', {
+      authorId: 'a.ai_42',
+      name: 'AI Assistant',
+      prefill: '@AI_Assistant ',
+    }, (out: any) => {
+      assert.equal(out, '@ai ');
+      done();
+    });
+  });
+
+  it('respects a custom trigger string', function (done) {
+    installWindowShim({ep_ai_chat: {trigger: '@bot', authorId: 'a.ai_99'}});
+    const {chatPrefillFromUser} = require('../../../../static/js/index');
+    chatPrefillFromUser('chatPrefillFromUser', {
+      authorId: 'a.ai_99',
+      name: 'AI Assistant',
+      prefill: '@AI_Assistant ',
+    }, (out: any) => {
+      assert.equal(out, '@bot ');
+      done();
+    });
+  });
+
+  it('falls back to default ("@ai ") if trigger is missing', function (done) {
+    installWindowShim({ep_ai_chat: {authorId: 'a.ai_42'}});
+    const {chatPrefillFromUser} = require('../../../../static/js/index');
+    chatPrefillFromUser('chatPrefillFromUser', {
+      authorId: 'a.ai_42', name: 'AI Assistant', prefill: '@AI_Assistant ',
+    }, (out: any) => {
+      assert.equal(out, '@ai ');
+      done();
+    });
+  });
+
+  it('returns nothing for a non-AI author so core uses its default', function (done) {
+    installWindowShim({ep_ai_chat: {trigger: '@ai', authorId: 'a.ai_42'}});
+    const {chatPrefillFromUser} = require('../../../../static/js/index');
+    chatPrefillFromUser('chatPrefillFromUser', {
+      authorId: 'a.alice_1', name: 'Alice', prefill: '@Alice ',
+    }, (out: any) => {
+      assert.equal(out, undefined);
+      done();
+    });
+  });
+
+  it('returns nothing when clientVars.ep_ai_chat is missing', function (done) {
+    installWindowShim({});
+    const {chatPrefillFromUser} = require('../../../../static/js/index');
+    chatPrefillFromUser('chatPrefillFromUser', {
+      authorId: 'a.ai_42', name: 'AI Assistant', prefill: '@AI_Assistant ',
+    }, (out: any) => {
+      assert.equal(out, undefined);
+      done();
+    });
+  });
+});