Add three tests to A2AServerServiceCollectionExtensionsTests that verify
custom keyed services are actually wired through to the A2AServer, not
just that the server resolves non-null:
- Custom IAgentHandler: verifies the keyed handler is invoked when
processing a SendMessageRequest instead of the default A2AAgentHandler.
- Custom AgentSessionStore (no handler): verifies the keyed session
store's GetSessionAsync is called during request processing when no
custom handler is registered.
- Default stores end-to-end: verifies the InMemoryAgentSessionStore and
InMemoryTaskStore defaults successfully process a request. Uses a new
CreateAgentMockForRequests helper that includes SerializeSessionCoreAsync
setup needed by InMemoryAgentSessionStore.
All tests call A2AServer.SendMessageAsync directly (no HTTP layer needed)
and use CancellationToken timeouts to guard against hangs.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>