Fix race in realtime tool-result unit test

[GarrettBeatty]

Problem

The Tests-arm64 action in the aws-dotnet-ai pipeline (Validation stage) failed with a single test failure:

Failed AWS.Bedrock.MEAI.BedrockRealtimeSessionTests.SendAsync_ConversationItemWithToolResult_SendsToolResult
  Assert.NotNull() Failure: Value is null
  at BedrockRealtimeSessionTests.cs:line 607
Total tests: 235   Passed: 234   Failed: 1

Root cause

This is a flaky test, not a product bug. BedrockNovaRealtimeSession.SendToolResultAsync writes three events in order to an unbounded channel — contentStart (TOOL) → toolResult → contentEnd — which a background single-reader task drains asynchronously into the test's capturedEvents list.

The test blocked only on the toolResult event, then immediately asserted that a contentEnd event also existed. Because contentEnd is written after toolResult and captured asynchronously, there is a window where toolResult is present but contentEnd is still in flight. On the arm64 runner that race was lost and the assertion saw null.

Fix

Wait for the trailing contentEnd event in addition to toolResult before running the assertions, closing the race. The session update in this test supplies no Instructions, so no earlier system-prompt contentEnd exists to match spuriously.

Testing

dotnet test for the affected test passes locally on net8.0 (the target that runs in the pipeline).