[Feature] Parallel Tool Call Execution

[da-daken]

Linked issue: #925

Purpose of change

Add parallel execution capability for tool calls via a new public API (RunnerContext.durableExecuteAllAsync()), which submits a batch of tool calls concurrently to an async thread pool and returns results in the original order after all complete.
The execution is durable: the state of in-flight calls is persisted, so that upon recovery (e.g., from checkpoint) the system correctly restores and completes the pending calls, ensuring no duplicate or lost invocations.

Tests

• Unit tests to verify the ordering and consistency of parallel execution results.
• Integration tests to validate correctness under concurrent scenarios.
• Fault-tolerance tests to ensure that after a simulated failure and recovery, the state of parallel executions is correctly restored and all results are eventually returned exactly once.

API

• Introduce a new public API RunnerContext.durableExecuteAllAsync() for batch parallel execution of tool calls.
• Expose the parallel execution capability without breaking compatibility with the existing synchronous API.

Documentation

• doc-needed
• doc-not-needed
• doc-included