mattermost: fix DM media upload for unprefixed user IDs (#29925)

Merged via squash.

Prepared head SHA: 5cffcb072c
Co-authored-by: teconomix <6959299+teconomix@users.noreply.github.com>
Co-authored-by: mukhtharcm <56378562+mukhtharcm@users.noreply.github.com>
Reviewed-by: @mukhtharcm

src/gateway/server-methods/send.ts

@@ -11,6 +11,7 @@ import {
 } from "../../infra/outbound/outbound-session.js";
 import { normalizeReplyPayloadsForDelivery } from "../../infra/outbound/payloads.js";
 import { buildOutboundSessionContext } from "../../infra/outbound/session-context.js";
+import { maybeResolveIdLikeTarget } from "../../infra/outbound/target-resolver.js";
 import { resolveOutboundTarget } from "../../infra/outbound/targets.js";
 import { normalizePollInput } from "../../polls.js";
 import {
@@ -194,6 +195,13 @@ export const sendHandlers: GatewayRequestHandlers = {
             meta: { channel },
           };
         }
+        const idLikeTarget = await maybeResolveIdLikeTarget({
+          cfg,
+          channel,
+          input: resolved.to,
+          accountId,
+        });
+        const deliveryTarget = idLikeTarget?.to ?? resolved.to;
         const outboundDeps = context.deps ? createOutboundSendDeps(context.deps) : undefined;
         const mirrorPayloads = normalizeReplyPayloadsForDelivery([
           { text: message, mediaUrl, mediaUrls },
@@ -225,7 +233,8 @@ export const sendHandlers: GatewayRequestHandlers = {
               channel,
               agentId: effectiveAgentId,
               accountId,
-              target: resolved.to,
+              target: deliveryTarget,
+              resolvedTarget: idLikeTarget,
               threadId,
             })
           : null;
@@ -246,7 +255,7 @@ export const sendHandlers: GatewayRequestHandlers = {
         const results = await deliverOutboundPayloads({
           cfg,
           channel: outboundChannel,
-          to: resolved.to,
+          to: deliveryTarget,
           accountId,
           payloads: [{ text: message, mediaUrl, mediaUrls }],
           session: outboundSession,