fix(typescript): add documented event helper methods

[realfishsam]

Summary

• Adds the documented TypeScript SDK Exchange methods preWarmMarket, getEventById, and getEventBySlug.
• Routes the methods through the sidecar method namespace, matching the existing Python SDK behavior.
• Adds focused Jest coverage for dispatch, null return handling, and event conversion.

Fixes #1491

Test Plan

• git diff --cached --check before commit
• npm test --workspace=pmxtjs -- event-methods.test.ts --runInBand was attempted but the checkout has no installed Node dependencies and npm install --workspaces --include-workspace-root timed out after 300s in this cron environment before producing node_modules; no test results are claimed from Jest.