Implement SlackAdapter satisfying the ChannelAdapter interface:
- ValidateConfig: rejects any webhook_url that doesn't start with
https://hooks.slack.com/ — returns "invalid Slack webhook URL" so
the handler surfaces 400 {"error":"invalid config: invalid Slack webhook URL"}
- SendMessage: HTTP POST JSON {"text":"..."} to the webhook URL with a
10s timeout; rejects invalid-prefix URLs at send time too (defence in depth)
- ParseWebhook: handles both slash-command (form-encoded) and Events API
(JSON) payloads; no-ops on url_verification and non-message events
- StartPolling: returns nil immediately (Slack doesn't support polling via
Incoming Webhooks)
Register "slack" in the adapter registry. Twelve unit tests cover
Type/DisplayName, happy-path validation, every bad-URL variant (wrong scheme,
wrong host, SSRF lookalike, empty string), empty webhook in SendMessage,
StartPolling nil return, and registry lookup/listing.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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