fix(gateway): make moderation score optional — LLM omits it in media batches

result.score was required by zod; the LLM (gemini-3.5-flash-lite via 9router)
occasionally omits it for media batches, hard-failing the whole batch parse
('Zod validation failed: expected number, received undefined' at
results[0].score). Callers already null-coalesce (result.score ?? 0) and the
parser clampScore()s it, so requiring it only caused parse failures.
Adds regression tests: media-batch without score parses (score->0), and
score-present responses still parse with the value.

services/discord-gateway/src/modules/ai-moderation/moderationSchemas.ts

@@ -23,7 +23,10 @@ export const ResultItemSchema = z.object({
   message_id: z.union([z.string(), z.number()]).transform(String),
   status: z.enum(["clean", "warn", "flagged"]),
   flags: z.array(z.string()).optional(),
-  score: z.number(),
+  // score is optional — the LLM occasionally omits it (especially media
+  // batches) and callers already null-coalesce (`result.score ?? 0`).
+  // Requiring it hard-fails the whole batch parse for one missing field.
+  score: z.number().optional(),
   analysis: z.string().nullable().optional(),
   categories: z.array(z.string()).optional(),
   severity: SeveritySchema.optional(),