feat(mcp): preserve MCP Apps result metadata

[dajiaohuang]

Summary

• preserve MCP Apps tool metadata and rich call-result fields separately from model-facing text
• carry structured app result data through tool progress events without expanding model context
• keep app-only tools out of the model-visible registry
• retain existing string results and non-WebUI fallback behavior

Draft scope

This is a backend-only first increment for early review. MCP extension capability advertisement, ui:// resource loading, the AppBridge, iframe hosting, and its security policy remain follow-ups and are not implied by this draft.

Relates to #5251.

Validation

• MCP and progress-event focused tests: 94 passed
• tools and utils test suites: passed through the exercised run
• Ruff: passed
• full-project BasedPyright: 0 errors, 0 warnings, 0 notes