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/llmChunkExtraction.test.ts

@@ -33,6 +33,41 @@ describe("extractChunkText — streaming chunk text extraction", () => {
     ).toBe("Task");
   });
 
+  it('falls back to delta.reasoning — mimo via 9router streams reasoning there with content:""', () => {
+    // Exact shape seen from 9router → mimo-v2.5-free (2026-08-11):
+    // {"choices":[{"delta":{"content":"","reasoning":"The user wants a","role":"assistant"},"finish_reason":null,...}]}
+    expect(
+      extractChunkText({
+        choices: [
+          {
+            delta: { content: "", reasoning: "The user wants a" },
+            finish_reason: null,
+          },
+        ],
+      }),
+    ).toBe("The user wants a");
+  });
+
+  it("joins delta.reasoning_details[].text when present", () => {
+    expect(
+      extractChunkText({
+        choices: [
+          {
+            delta: {
+              content: "",
+              reasoning: "",
+              reasoning_details: [
+                { type: "reasoning.text", text: " detailed", index: 0 },
+                { type: "reasoning.text", text: " description", index: 1 },
+              ],
+            },
+            finish_reason: null,
+          },
+        ],
+      }),
+    ).toBe(" detailed description");
+  });
+
   it("prefers content over reasoning when both present (deepseek-style final answer)", () => {
     expect(
       extractChunkText({