State serialization can collapse distinct dictionary keys

### Feature Summary

State serialization converts every dictionary key to a string. Distinct Python keys can therefore collapse to the same JSON key and silently overwrite state.

Before: integer key 1 and string key 1 -> one stored value survives

After: any non-string state key -> TypeError before serialization

Reproduction evidence:

Round-trip state containing a nested dictionary with integer key 1 and string key 1.

Version and commit evidence:

1.3.0-incubating-SNAPSHOT (main)

**Commit Hash**

70c21145887920528d7d5540e3fb790b43e8b759

**What browsers are you seeing the problem on?**

Not browser-specific.

**Relevant log output**

```text
original= {'config': {1: 'integer key', '1': 'string key'}}
encoded= {"config":{"1":"string key"}}
round_trip_equal= False
```

### Proposed Solution or Design

After: any non-string state key -> TypeError before serialization

### Affected Area

Workflow Engine (Amber)