TypeError: _a.trim is not a function during exposure event

## Expected Behavior

The Experiment Node Server SDK should safely handle non-string values when canonicalizing exposure data before calling `.trim()`. Invalid or unexpected input types should not cause a runtime exception during variant evaluation or exposure tracking.

## Current Behavior

A runtime exception is thrown during exposure tracking:

````
TypeError _a.trim is not a function
/app/node_modules/@amplitude/experiment-node-server/dist/src/exposure/exposure.js:16:93 Exposure.canonicalize
/app/node_modules/@amplitude/experiment-node-server/dist/src/exposure/exposure-filter.js:16:44 InMemoryExposureFilter.shouldTrack
/app/node_modules/@amplitude/experiment-node-server/dist/src/exposure/exposure-service.js:23:37 AmplitudeExposureService.
/app/node_modules/@amplitude/experiment-node-server/dist/src/exposure/exposure-service.js:8:71
/app/node_modules/@amplitude/experiment-node-server/dist/src/exposure/exposure-service.js:4:12 __awaiter
/app/node_modules/@amplitude/experiment-node-server/dist/src/exposure/exposure-service.js:22:16 AmplitudeExposureService.track
/app/node_modules/@amplitude/experiment-node-server/dist/src/local/client.js:149:87 LocalEvaluationClient.evaluateV2
.next/server/chunks/7651.js:19:1755 d.fetchVariants
````

## Possible Solution

Improve input validation earlier in the exposure pipeline.

## Steps to Reproduce

We weren't able to reproduce it locally.

## Environment
- JS Server SDK Version: 1.13.2
- Runtime: Node.js
- Installation Method: yarn
- Browser and Version: N/A