fix(discord): autoThread ack reactions + exec approval null handling (#1511)

* fix(discord): gate autoThread by thread owner

* fix(discord): ack bot-owned autoThreads

* fix(discord): ack mentions in open channels

- Ack reactions in bot-owned autoThreads
- Ack reactions in open channels (no mention required)
- DRY: Pass pre-computed isAutoThreadOwnedByBot to avoid redundant checks
- Consolidate ack logic with explanatory comment

* fix: allow null values in exec.approval.request schema

The ExecApprovalRequestParamsSchema was rejecting null values for optional
fields like resolvedPath, but the calling code in bash-tools.exec.ts passes
null. This caused intermittent 'invalid exec.approval.request params'
validation errors.

Fix: Accept Type.Union([Type.String(), Type.Null()]) for all optional string
fields in the schema. Update test to reflect new behavior.

* fix: align discord ack reactions with mention gating (#1511) (thanks @pvoo)

---------

Co-authored-by: Wimmie <wimmie@tameson.com>
Co-authored-by: Peter Steinberger <steipete@gmail.com>

src/discord/monitor/message-handler.preflight.ts

@@ -328,9 +328,12 @@ export async function preflightDiscordMessage(
         } satisfies HistoryEntry)
       : undefined;
 
+  const threadOwnerId = threadChannel ? (threadChannel.ownerId ?? channelInfo?.ownerId) : undefined;
   const shouldRequireMention = resolveDiscordShouldRequireMention({
     isGuildMessage,
     isThread: Boolean(threadChannel),
+    botId,
+    threadOwnerId,
     channelConfig,
     guildInfo,
   });