fix(slack): validate webhook timestamps strictly

[aiirvizionz]

Summary

• require Slack request signature timestamps to be decimal Unix-second integers
• reject exponent, hexadecimal, fractional, signed, and whitespace-padded forms
• reject values outside JavaScript safe-integer range before replay-window checks

Validation

• vitest run packages/bots/slack/src/index.test.ts (10 tests passed)
• tsc -p packages/bots/slack/tsconfig.json --noEmit
• git diff --check

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🤖 Auto-rebase: The branch was rebased successfully locally but could not be pushed to the fork. Please enable 'Allow edits from maintainers' in the PR settings, or rebase manually: git fetch upstream master && git rebase upstream/master.