Investigate non-string serverLogs: strengthen ErrorReportData type

## Background

During review of #11268 (regression test for #8460), it was noted that `ErrorReportData.serverLogs` is currently typed as `string` in `src/utils/errorReportUtil.ts`, but the production code already has a runtime guard:

```ts
typeof error.serverLogs === 'string' ? ... : JSON.stringify(...)
```

This indicates `serverLogs` can in practice be a non-string value (e.g., an object). Fixing the type was considered out of scope for #11268 which only adds regression tests.

## Task

- Investigate what server messages or conditions produce a non-string `serverLogs` value (e.g., object-shaped log entries).
- Determine the correct type (e.g., `unknown`, `string | object`, or a more precise schema).
- Update `ErrorReportData.serverLogs` to reflect the real type and remove any unsafe casts in call-sites.

## References

- PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/11268
- Review comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/11268#discussion_r3087889280

Requested by @pythongosssss

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