MCP Apps: Add round-trip integration test for _meta.ui serialization

[mikekistler]

From PR #1484 review comment by @halter73:

No round-trip integration test that goes through McpClient.ListToolsAsync() and verifies _meta.ui survives serialization end-to-end (e.g. via ClientServerTestBase). All current tests are pure unit tests on the option pipeline.

Add an integration test (using ClientServerTestBase) that registers a tool with [McpAppUi] metadata, calls ListToolsAsync() on the client, and verifies the _meta.ui object is present and correctly shaped after round-tripping through serialization.

[yayayouyou]

Hi, I'm yayou 👋 — picking this one up. Implemented exactly per the spec in the issue: ClientServerTestBase-based tests registering [McpAppUi] tools behind WithTools<T>().WithMcpApps(), listing via McpClient.ListToolsAsync(), and asserting the structured _meta.ui shape after the wire round-trip (resourceUri, visibility present when restricted / absent when not, and no ui key for unattributed tools). PR incoming.