Expand API call coverage across bridge, host, and eval runtime

[zac]

Summary

• Extend bridge model and error handling so more API calls are represented and validated consistently
• Broaden host/runtime wiring for capability loading, JavaScript bindings, and platform-specific system UI presentation
• Update evaluation scenarios and tool descriptions to exercise the expanded call surface
• Add coverage for registry, host configuration, runtime, and system UI bridge behavior

Testing

• Added and updated unit tests for registry, host validation, runtime, and system UI bridging paths
• Not run (not requested)