refactor(ai-chat)!: unify provider error formatting via describeProviderError

Behaviour change (split out of the test commit per review, and now covered).

Both the stream onError log line and the error text streamed to the client were
formatted by separate inline blocks that only emitted "<status>: <message>".
Route both through the shared describeProviderError() so formatting stays in one
place.

BEHAVIOUR CHANGE: describeProviderError additionally appends a single-line,
300-char-truncated snippet of the provider responseBody/text. So the log line
AND the user-facing stream error now include that snippet (e.g. the HTML error
page from a misconfigured endpoint), which previously neither did. This is
intentional — it makes a misconfigured external endpoint diagnosable — and is
safe: the API key travels in the Authorization header and is never echoed in
the response body (see the util's docstring). A `fallback` param is added so
each call site keeps its own default ('AI stream error' for the stream).

Adds ai-error.util.spec.ts covering the formatter, including the appended /
truncated body snippet, so this behaviour is no longer untested.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/server/src/core/ai-chat/ai-chat.service.ts

@@ -10,6 +10,7 @@ import {
 } from 'ai';
 import { AiService } from '../../integrations/ai/ai.service';
 import { AiSettingsService } from '../../integrations/ai/ai-settings.service';
+import { describeProviderError } from '../../integrations/ai/ai-error.util';
 import { AiChatRepo } from '@docmost/db/repos/ai-chat/ai-chat.repo';
 import { AiChatMessageRepo } from '@docmost/db/repos/ai-chat/ai-chat-message.repo';
 import { User, Workspace, AiChatMessage } from '@docmost/db/types/entity.types';
@@ -271,15 +272,10 @@ export class AiChatService {
       onError: async ({ error }) => {
         // NestJS Logger.error(message, stack?, context?): pass the real message
         // (with statusCode when present) + the stack string, not the Error
-        // object, so the actual provider cause is clearly logged.
-        const e = error as {
-          statusCode?: number;
-          message?: string;
-          stack?: string;
-        };
-        const errorText = e?.statusCode
-          ? `${e.statusCode}: ${e.message ?? String(error)}`
-          : (e?.message ?? String(error));
+        // object, so the actual provider cause is clearly logged. Reuse the
+        // shared formatter so provider error formatting stays unified.
+        const e = error as { stack?: string };
+        const errorText = describeProviderError(error, String(error));
         this.logger.error(`AI chat stream error: ${errorText}`, e?.stack);
         // Persist whatever text we have (likely empty) so the turn is recorded,
         // and record the error text in metadata so it is visible in history.
@@ -340,10 +336,9 @@ export class AiChatService {
       result.pipeUIMessageStreamToResponse(res.raw, {
         headers: { 'X-Accel-Buffering': 'no' },
         onError: (error: unknown) => {
-          const e = error as { statusCode?: number; message?: string };
-          return e?.statusCode
-            ? `${e.statusCode}: ${e.message}`
-            : (e?.message ?? 'AI stream error');
+          // Reuse the shared formatter so provider error formatting stays
+          // unified between the log line and the streamed error message.
+          return describeProviderError(error, 'AI stream error');
         },
       });
 
@@ -538,8 +533,8 @@ function compactValue(value: unknown, depth: number): unknown {
  * recovers the name. Falls back to a single `text` part built from
  * `fallbackText` when the steps carry no text.
  */
-// Exported only so the unit tests can import this pure helper; exporting it
-// does not change runtime behavior.
+// Exported only so the unit tests can import these pure helpers; exporting
+// them does not change runtime behavior.
 export function assistantParts(
   steps: ReadonlyArray<StepLike> | undefined,
   fallbackText: string,