fix: require visible chat output before callback delivery

[seven332]

Summary

• Add a shared completed chat output resolver that reads DB materialized output first and falls back to watermark-gated Axiom output events.
• Route Slack, Teams, Telegram, and AgentPhone completion callbacks through the resolver so transient output invisibility fails the callback instead of posting a misleading fallback or footer-only response.
• Keep confirmed-empty completions explicit and add route-level regression coverage for delayed Slack output visibility and non-Slack completion paths.

Closes #20643

Verification

• pnpm -F api exec vitest run src/signals/routes/__tests__/internal-callbacks-teams.test.ts
• pnpm -F api exec vitest run src/signals/routes/__tests__/internal-callbacks-telegram.test.ts
• pnpm -F api exec vitest run src/signals/routes/__tests__/agentphone.bdd.test.ts
• pnpm -F api exec vitest run src/signals/routes/__tests__/integrations.bdd.test.ts -t "waits for Slack completed output|delivers Slack org callbacks"
• pnpm -F api exec vitest run src/signals/routes/__tests__/integrations.bdd.test.ts -t "runs Slack mentions|starts a new Slack session"
• pnpm -F api lint
• pnpm -F api check-types
• pnpm knip --cache
• pre-commit turbo run check-types --concurrency=1

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
4375	1	4374	11

View the top 1 failed test(s) by shortest run time

apps/api/src/signals/routes/__tests__/test-slack-dispatch-probe.test.ts > POST /api/test/slack-dispatch-probe > routes preview Slack Web API calls to API mock routes

Stack Traces | 0.349s run time

AssertionError: expected "vi.fn()" to be called with arguments: [ Any<String>, { …(4) } ]

Received:

  1st vi.fn() call:

  [
-   Any<String>,
+   "xoxb-e2e-test-bot-token",
    {
      "headers": {
        "x-vercel-protection-bypass": "preview-secret",
        "x-vm0-test-endpoint-bypass": "preview-secret",
      },
      "retryConfig": {
        "retries": 1,
      },
      "slackApiUrl": "https://pr-13948-api.vm6..../api/test/slack-mock/",
-     "timeout": 5000,
+     "timeout": 15000,
    },
  ]


Number of calls: 1

 ❯ .../routes/__tests__/test-slack-dispatch-probe.test.ts:434:23

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[github-actions]

API preview: https://pr-20704-api.vm6.ai

[seven332]

Closing per maintainer decision; this approach will not be pursued.