fix(ai-moderation): never cache vision outputs that claim 'no image seen'

Root cause (3rd layer after 50371bd + 4f4c435): a vision model run
(2026-08-10) returned 'Maaf, saya tidak melihat gambar apapun yang terlampir...'
and that text was cached as a VALID vision_llm result (image + phash keys,
24h/7d TTL). Every subsequent analysis of the same image (same hash/phash)
hit the poisoned cache, so image analysis looked broken forever even though
9router responded fine — the moderation LLM wrote 'lampiran yang gagal
terbaca' from a cache hit.

Also: mimo via 9router streams reasoning in delta.reasoning +
delta.reasoning_details[].text (content:"") — extractChunkText only read
delta.reasoning_content, so those runs aggregated empty → 'Vision API null
response' (observed 08:54/09:07/09:38).

Fixes:
- llmClient.extractChunkText: fall back to delta.reasoning and
  reasoning_details[].text (mimo), on top of reasoning_content (gemma).
- visionAnalyzer: isNoImageSeenText() detects 'no image' style outputs;
  such results are NEVER cached, and poisoned entries are purged when hit
  (LRU/DB/phash) so re-analysis actually re-runs vision.
- Tests: reasoning/reasoning_details extraction + isNoImageSeenText
  (Indonesian + English, no false positives on real descriptions).

services/discord-gateway/tests/visionNoImageSeen.test.ts

@@ -0,0 +1,52 @@
+// ═══════════════════════════════════════════════════════════════════════════
+// isNoImageSeenText — vision outputs that claim "no image" must not be cached
+// ═══════════════════════════════════════════════════════════════════════════
+// Regression (2026-08-11): the vision model sometimes answered "Maaf, saya
+// tidak melihat gambar apapun yang terlampir..." and that text was cached as
+// a VALID vision_llm result. Every later analysis of the same image (same
+// hash / phash) then hit the poisoned cache and the moderation LLM wrote
+// "lampiran yang gagal terbaca" — image analysis seemed permanently broken
+// even though 9router was responding fine.
+import { describe, expect, it } from "vitest";
+import { isNoImageSeenText } from "../src/modules/ai-moderation/visionAnalyzer.js";
+
+describe("isNoImageSeenText — poisoned vision output detection", () => {
+  it("detects the exact poisoned strings seen in production", () => {
+    expect(
+      isNoImageSeenText(
+        "Maaf, saya tidak melihat gambar apapun yang terlampir dalam pesan Anda. Mohon kirimkan ulang gambarnya agar saya bisa mendeskripsikannya secara objektif dan spesifik.",
+      ),
+    ).toBe(true);
+    expect(
+      isNoImageSeenText(
+        "Tidak ada gambar yang terlampir. Tidak bisa deskripsi tanpa input visual.",
+      ),
+    ).toBe(true);
+  });
+
+  it("detects English variants", () => {
+    expect(isNoImageSeenText("I cannot see any image in this message")).toBe(
+      true,
+    );
+    expect(isNoImageSeenText("No image provided")).toBe(true);
+    expect(isNoImageSeenText("there is no image attached")).toBe(true);
+    expect(isNoImageSeenText("I don't see an image")).toBe(true);
+  });
+
+  it("does NOT flag legitimate image descriptions", () => {
+    expect(
+      isNoImageSeenText(
+        "Gambar ini menampilkan dua panel komik, seorang gadis berambut biru tersipu saat dipuji.",
+      ),
+    ).toBe(false);
+    expect(
+      isNoImageSeenText("Ini adalah screenshot dari sebuah website rekrutmen."),
+    ).toBe(false);
+    expect(isNoImageSeenText("Emoji menampilkan ekspresi wajah tertawa.")).toBe(
+      false,
+    );
+    expect(isNoImageSeenText(null)).toBe(false);
+    expect(isNoImageSeenText(undefined)).toBe(false);
+    expect(isNoImageSeenText("")).toBe(false);
+  });
+});