fix(strands-py-wasm): access structured-output via kebab-case attr in invoke_async

[pgrayy]

Description

The _AgentResultAccumulator.finalize() method accessed stop.structured_output using Python dot notation, but the StopEvent from the WASM runtime is a raw wasmtime Record that stores the field under its WIT kebab-case name structured-output. This caused an AttributeError on every invoke_async call.

Uses getattr(stop, "structured-output") to access the kebab-case attribute correctly.

Also adds an invoke_async integration test.

Related Issues

Tracking: strands-agents/sdk-typescript#1102

Type of Change

Bug fix

Testing

• Added test_invoke_async_hello_world integration test that exercises invoke_async end-to-end
• Manually verified concurrent invoke_async calls via asyncio.gather achieve ~1.7x speedup over sequential execution

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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