fix: await async custom dispatcher during telemetry sync

[danishsshaikh]

Summary

Fixes #62.

This PR allows custom telemetry dispatchers to return a Promise and ensures async dispatcher failures are handled by the existing retry flow.

Problem

The SDK supports custom dispatchers, but TelemetrySyncManager called dispatcher.dispatch(...) without await. If a custom dispatcher returned a rejected Promise, the surrounding try/catch would not catch the error, so failed batches could bypass retry handling.

Changes

• Updated the custom dispatcher type to allow void | Promise<unknown>.
• Awaited custom dispatcher calls in normal sync and retry sync paths.
• Added a regression test for rejected async custom dispatchers.

Validation

• npm test — 36 tests passed
• npm run build — CJS, ESM, IIFE, and DTS builds passed
• npm run lint — completed with no lint errors