fix: let channels control reply length

ChannelService previously truncated every reply at 4,000 characters and discarded the remainder before the platform adapter could handle it. Complete executor output now reaches each channel driver, while error redaction and channel-specific platform limits remain intact.

Release note: 消息通道回复不再由公共服务统一截断；完整结果现在交由微信、企业微信或钉钉按各自平台能力处理。

src/shared/channel-contracts.ts

@@ -5,7 +5,6 @@ export const CHANNEL_LIMITS = {
   maximumEventIdLength: 256,
   maximumIdentityLength: 256,
   maximumTextLength: 32_000,
-  maximumResultLength: 16_000,
   maximumErrorLength: 1_000,
   maximumStatusLength: 64,
   maximumAttachmentCount: 4,
@@ -185,10 +184,7 @@ export const channelResultMessageSchema = z
       .trim()
       .min(1)
       .max(CHANNEL_LIMITS.maximumStatusLength),
-    output: z
-      .string()
-      .max(CHANNEL_LIMITS.maximumResultLength)
-      .optional(),
+    output: z.string().optional(),
     error: z
       .string()
       .max(CHANNEL_LIMITS.maximumErrorLength)