fix(slack): return replyable thread ID from channel posts

[akkadaska]

summary

fixes #719

updates Slack channel posts to return a replyable thread ID rooted at the newly posted top-level message

postChannelMessage still omits thread_ts for the initial channel post, then uses Slack's returned message ts to construct slack:<channel>:<ts> for the SentMessage

this allows chat.thread(sent.threadId).post(...) to reply to a message returned by channel.post(...) instead of creating another top-level channel message

file-only uploads without a Slack message ts keep their existing channel-scoped thread ID

test plan

• added a Slack adapter regression assertion that postChannelMessage returns slack:C123:<message ts>
• ran the @chat-adapter/slack test suite
• ran pnpm validate

checklist

• all commits are signed and verified
• targeted builds and tests pass
• changeset added or not applicable
• documentation updated or not applicable

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