fix(adk): only compact MCP App results that carry a render payload

[wangzhengzhuo05]

Summary

MakeMCPAppModelResultCallback replaced the result of every MCP App-capable tool with the canned render notice, even when an individual call returned an ordinary text-only result (no structuredContent, no _meta.ui). That hid real content from the model.

The callback now collapses a successful result only when the result itself carries a render payload — a non-nil structuredContent or a _meta.ui.resourceUri — and passes plain results through unchanged. This mirrors the Python-side fix in #2579 so Go and Python stop drifting on MCP App semantics.

What changed

• go/adk/pkg/mcp/mcp_ui.go: add an exported HasUIResource(meta) helper that reuses the existing parseMCPUIMetadata as the single source of truth for _meta.ui parsing.
• go/adk/pkg/agent/mcp_apps.go: gate the success-path compaction on result.StructuredContent == nil && !kagentmcp.HasUIResource(result.Meta); return the response unchanged when there is no render payload. The isError path keeps its existing behaviour.
• go/adk/pkg/agent/mcp_apps_test.go: add a regression test using the issue's repro — an app-declared tool returning only content: [{type: text, text: "No build found for job demo/1: it was deleted."}] must reach the model unchanged.

Why not just branch on IsError

A tool declaring _meta.ui.resourceUri is capable of rendering an app; a given call may still return plain content (e.g. "not found"). Deciding from the tool name alone conflates capability with an actual render payload, which is what the regression covers.

Verification

• go test ./adk/pkg/agent/... -count=1 — pass (4 MCP App tests: 3 pre-existing + 1 new)
• go test ./adk/pkg/mcp/... -count=1 — pass
• go build ./... — pass
• Mutation check: with the new guard disabled, TestMakeMCPAppModelResultCallbackPassesThroughPlainResultFromAppTool fails exactly as expected (the plain text is replaced by the render notice). Restored afterwards.

Fixes #2716

[wangzhengzhuo05]

Closing this to keep the tracker clean \u2014 it has been sitting without review for several days. This is no reflection on the change itself: the fix and its local verification still stand. If the issue is still present and you would like the patch revisited, say the word and I will rebase it onto current main and re-open. Thanks for your time.