fix(slack): guard invalid timestamp tolerance config

[aiirvizionz]

Summary

• Keep Slack request timestamp validation active when timestampToleranceSeconds receives invalid numeric values.
• Preserve the explicit 0 escape hatch for disabling timestamp checks, while falling back to the default 300 seconds for NaN, negative, or otherwise invalid values.
• Add a regression test that sends a stale but correctly signed Slack Events request with Number.NaN config and expects rejection.

Validation

• git diff --check

I attempted pnpm vitest run packages/bots/slack/src/index.test.ts and pnpm --filter @profullstack/sh1pt-bot-slack typecheck, but this checkout stops during dependency preparation because the committed lockfile fails the active supply-chain policy: @profullstack/autoblog@0.4.0 has no tarball integrity field. I did not modify the lockfile; the regression test is included for maintainers' normal CI/runtime.

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