[WASM] Integration tests: Basic agent invocation

[awsarron]

Parent issue

strands-agents/sdk-typescript#1104

Scope

Write integration tests validating the core WASM runtime bridge for basic agent operations:

• agent.invoke("prompt") - synchronous invocation
• agent.invoke_async("prompt") - async invocation
• agent.stream_async("prompt") - streaming with event iteration
• Cancellation via agent.cancel()
• AgentResult structure validation (stop_reason, last_message, usage)

Context

This is foundational - all other test issues depend on this working. Validates:

• wasmtime-py call_async component model bridge
• Event stream pull-based read loop
• Result accumulation from stream events

Done when

• Tests pass for invoke, invoke_async, and stream_async
• Cancellation test demonstrates clean shutdown
• AgentResult fields are correctly populated from stream events
• Tests run against at least one model provider (Bedrock default)