[Samples] Migrate A2UI in MCP Apps agent to A2UI v0.9

[jacobsimionato]

Update the a2ui-in-mcpapps server sample to output v0.9 A2UI messages.

[jacobsimionato]

Coding agent prompt:
Design Document: Migrate the agent in @samples/agent/mcp/a2ui-in-mcpapps/server/ to A2UI v0.9.

Strategy:

1. Update simple_counter_a2ui.json to use the v0.9 schema (replacing beginRendering with createSurface, etc.).
2. Update the server logic in server.py if it performs any manual protocol-specific assembly.
3. Ensure the associated client in the same directory is also aware of the version change.

Context:

• @samples/agent/mcp/a2ui-in-mcpapps/
• @specification/v0_9/

[jacobsimionato]

Any chance you could take this on, Mandar?