[WASM] Integration tests: App state and model state

[awsarron]

Parent issue

strands-agents/sdk-typescript#1104

Scope

Write integration tests validating state management across invocations:

• agent.get_app_state() / agent.set_app_state() - arbitrary JSON state
• agent.get_model_state() / agent.set_model_state() - provider-specific state
• State persists across multiple invocations on same agent instance
• State round-trips correctly through WIT (serialization fidelity)
• Session save/load preserves state

Context

App state and model state are get/set via WIT resource methods on the agent. They cross the WASM boundary as JSON-compatible values. Session managers can persist these alongside messages.

Key areas:

• Complex nested JSON structures survive the WIT round-trip
• State is accessible between invocations without re-initialization
• Session save captures state, session load restores it

Done when

• App state set/get works with primitive and nested values
• Model state set/get works
• State persists across multiple invocations
• Session save/load preserves both state types
• Edge cases: empty state, large state, null values