fix(slack): enforce message size limits

[Zygimantass]

Summary

• Add Slack-compatible top-level text truncation for chat writes and incoming webhooks at 40,000 characters.
• Return response_metadata.warnings: ["message_truncated"] on Web API chat responses when text is truncated.
• Reject oversized rich payloads, including more than 50 blocks, overlong section/context/header/markdown block text, and more than 100 attachments.
• Add a 10 MB @slack/web-api regression test for chat.postMessage.

Tests

• pnpm --filter @emulators/slack test
• pnpm --filter @emulators/slack type-check
• pnpm --filter @emulators/slack lint
• git diff --check
• pnpm exec prettier --check apps/web/app/docs/slack/page.mdx packages/@emulators/slack/src/helpers.ts packages/@emulators/slack/src/routes/chat.ts packages/@emulators/slack/src/routes/webhooks.ts packages/@emulators/slack/src/__tests__/slack-sdk.test.ts packages/@emulators/slack/src/__tests__/slack.test.ts

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