fix(slack): reject malformed event timestamps

[aiirvizionz]

Summary

• Tightens Slack event timestamp parsing so malformed values like .000200 do not become Unix epoch timestamps
• Falls back to the current time for malformed or invalid Slack timestamp values
• Adds Socket Mode regression coverage for malformed event timestamps

Verification

• node_modules/.bin/vitest.CMD run packages/bridges/slack/src/index.test.ts
• node_modules/.bin/tsc.CMD -p packages/bridges/slack/tsconfig.json --noEmit
• git diff --check