slack: schedule messages instead of dropping them on rate limit

[nazriz]

Problem

During alert bursts the notifier can exceed Slack’s per-channel chat.postMessage rate limit (~1 message/sec). Slack responds with HTTP 429, and these responses are currently treated as runtime errors, causing alerts to be dropped.

Solution

Expose Slack’s scheduled message APIs and introduce an optional rate-limit mode where postMessage returns a structured ratelimited response instead of failing.

This allows the PagerDuty Slack notifier to own the retry policy by scheduling messages into future delivery windows, retrying if a scheduling window is full, while keeping the runtime Slack-agnostic.

Tested with a simulated 300-message alert storm resulted in zero dropped alerts and no runtime error logs.

[Copilot]

Pull request overview

This PR improves Slack alert delivery during burst conditions by falling back from chat.postMessage to chat.scheduleMessage when Slack returns HTTP 429, and adds new runtime/STL wrappers to support cancelling scheduled messages and retrieving message history (with metadata) for correlation.

Changes:

• Add a postMessage 429 fallback that schedules messages into the earliest available 5‑minute window (with retries for restricted_too_many and schedule API 429s).
• Add runtime API endpoints for chat.deleteScheduledMessage and conversations.history.
• Extend the Plaid STL Slack module with new types and helper functions for scheduled delivery responses, scheduled-message deletion, and history retrieval.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
runtime/plaid/src/functions/api.rs	Registers new Slack host functions for scheduled-message deletion and conversations history.
runtime/plaid/src/apis/slack/api.rs	Implements 429→schedule fallback, adds Slack API request builders for the new endpoints, and includes unit tests for scheduling helpers.
runtime/plaid-stl/src/slack/mod.rs	Adds new STL request/response types and wrappers for scheduled-message deletion and conversations history, plus a new post helper that can represent scheduled delivery.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

[obelisk]

I think this scheduling should be done at the rule level letting us more quickly respond to changes in the Slack API and keeping the runtime simpler.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

[nazriz]

I believe the check failed due to network error, job needs to be re-run, but I am unable to do so.