[Fix] Automations fail to report when Roomote cannot access Slack channel

[roomote-roomote]

Opened on behalf of Matt Rubens. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

What changed

• Check that the Slack app can access a custom automation's configured report channel before launching its task.
• Record a clear automation failure that tells an admin to invite the app or choose another channel.
• Preserve Slack's actual API error code when a late-bound report thread cannot create its root message.

Why this change was made

A custom automation targeting a channel the Slack app could not access still launched and completed its work, then repeatedly failed to deliver the result. The underlying not_in_channel or channel_not_found response was discarded and surfaced as a misleading generic 502 about a missing message timestamp.

Impact

Inaccessible Slack report destinations now fail before consuming task compute, and any remaining root-post failures identify the real Slack API error for faster remediation.

[roomote-community]

1 issue outstanding. See task

• packages/sdk/src/server/automations/__tests__/custom-automations.test.ts:40-44 The mocked Slack adapter has no provider: 'slack', so the new guard treats it as unverifiable even when isAppInChannelMock resolves true. This prevents enqueueTask in the normal Slack cases and leaves eight tests failing; include the discriminator in the mock.

_{Reviewed f5d0662}