fix(mattermost): prevent duplicate messages when block streaming + threading are active (#41362)

* fix(mattermost): prevent duplicate messages when block streaming + threading are active

Remove replyToId from createBlockReplyPayloadKey so identical content is
deduplicated regardless of threading target. Add explicit threading dock
to the Mattermost plugin with resolveReplyToMode reading from config
(default "all"), and add replyToMode to the Mattermost config schema.

Fixes #41219

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix(mattermost): address PR review — per-account replyToMode and test clarity

Read replyToMode from the merged per-account config via
resolveMattermostAccount so account-level overrides are honored in
multi-account setups. Add replyToMode to MattermostAccountConfig type.
Rename misleading test to clarify it exercises shouldDropFinalPayloads
short-circuit, not payload key dedup.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Replies: keep block-pipeline reply targets distinct

* Tests: cover block reply target-aware dedupe

* Update CHANGELOG.md

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Vincent Koc <vincentkoc@ieee.org>

extensions/mattermost/src/channel.ts

@@ -270,6 +270,16 @@ export const mattermostPlugin: ChannelPlugin<ResolvedMattermostAccount> = {
   streaming: {
     blockStreamingCoalesceDefaults: { minChars: 1500, idleMs: 1000 },
   },
+  threading: {
+    resolveReplyToMode: ({ cfg, accountId }) => {
+      const account = resolveMattermostAccount({ cfg, accountId: accountId ?? "default" });
+      const mode = account.config.replyToMode;
+      if (mode === "off" || mode === "first") {
+        return mode;
+      }
+      return "all";
+    },
+  },
   reload: { configPrefixes: ["channels.mattermost"] },
   configSchema: buildChannelConfigSchema(MattermostConfigSchema),
   config: {