feat(ai-assist): pure prompt builder + result applier

buildAssistPrompt(action, selection) returns the {system, messages}
payload for rewrite/shorten/expand. Enforces 8KB selection cap and
typed errors with codes (unknown_action, bad_selection,
empty_selection, selection_too_large) so the IPC layer can map them
to user-friendly messages.

applyAssistResult(original, newText) trims the response and rejects
empty/unchanged payloads so a no-op streaming chunk doesn't churn
the editor.

15 new tests cover action coverage, prompt content, all four error
codes, cap-edge (exactly 8KB OK, 8KB+1 rejected), and trim semantics.

Amit Haridas

src/ai-assist/inline-assist.js

@@ -0,0 +1,107 @@
+/**
+ * Inline AI assist — prompt builder + result applier.
+ *
+ * Pure module. The actual network call goes through AiProviders.js; this
+ * module shapes the request (system + user messages) and validates the
+ * response before it reaches the editor.
+ *
+ * Actions (v4.13.0): rewrite / shorten / expand.
+ *
+ * Safety: MAX_SELECTION_CHARS caps the selection so a runaway or hostile
+ * caller can't push an 8KB+ blob at an API. The cap is enforced BEFORE
+ * the IPC hop so the main process sees only safe payloads.
+ *
+ * @module inline-assist
+ */
+
+const MAX_SELECTION_CHARS = 8 * 1024;
+
+const ACTIONS = {
+  rewrite: {
+    label: 'Rewrite',
+    systemPrompt:
+      'You are a precise writing assistant. Rewrite the selected text to improve clarity, ' +
+      'flow, and word choice while preserving the original meaning, tone, and length. ' +
+      'Return ONLY the rewritten text — no preamble, no explanation, no surrounding quotes.',
+  },
+  shorten: {
+    label: 'Shorten',
+    systemPrompt:
+      'You are a precise writing assistant. Shorten the selected text while preserving all ' +
+      'key information. Aim for 30–50% reduction in length. Drop filler, redundancy, and ' +
+      'verbose phrasing. Return ONLY the shortened text — no preamble, no explanation.',
+  },
+  expand: {
+    label: 'Expand',
+    systemPrompt:
+      'You are a precise writing assistant. Expand the selected text by adding relevant ' +
+      'detail, examples, or explanation. Aim for 50–100% expansion. Stay on-topic. ' +
+      'Return ONLY the expanded text — no preamble, no explanation.',
+  },
+};
+
+const ACTIONS_LIST = Object.keys(ACTIONS);
+
+/**
+ * Build the {system, messages} payload for `action`.
+ *
+ * @param {'rewrite'|'shorten'|'expand'} action
+ * @param {string} selection
+ * @returns {{system:string, messages:Array<{role:'user', content:string}>}}
+ * @throws when action is unknown or selection is empty / too large
+ */
+function buildAssistPrompt(action, selection) {
+  const def = ACTIONS[action];
+  if (!def) {
+    const err = new Error(`Unknown AI assist action: ${action}`);
+    err.code = 'unknown_action';
+    throw err;
+  }
+  if (typeof selection !== 'string') {
+    const err = new Error('Selection must be a string');
+    err.code = 'bad_selection';
+    throw err;
+  }
+  if (selection.length === 0) {
+    const err = new Error('Selection is empty');
+    err.code = 'empty_selection';
+    throw err;
+  }
+  if (selection.length > MAX_SELECTION_CHARS) {
+    const err = new Error(
+      `Selection is too large (${selection.length} chars; max ${MAX_SELECTION_CHARS})`
+    );
+    err.code = 'selection_too_large';
+    throw err;
+  }
+  return {
+    system: def.systemPrompt,
+    messages: [{ role: 'user', content: selection }],
+  };
+}
+
+/**
+ * Validate the provider's response before applying it to the editor.
+ *
+ * - Empty / whitespace-only → return null (don't replace selection with nothing)
+ * - Unchanged → return null (no-op, don't churn the editor)
+ *
+ * @param {string} originalText - The original selection
+ * @param {string} newText - The provider's response
+ * @returns {string|null} text to apply, or null to skip
+ */
+function applyAssistResult(originalText, newText) {
+  if (typeof newText !== 'string') return null;
+  const trimmed = newText.trim();
+  if (trimmed.length === 0) return null;
+  if (trimmed === String(originalText || '').trim()) return null;
+  return trimmed;
+}
+
+module.exports = {
+  buildAssistPrompt,
+  applyAssistResult,
+  ACTIONS,
+  ACTIONS_LIST,
+  MAX_SELECTION_CHARS,
+};