Support Rizzcharts example from internal Google infrastructure

[jonsharkey]

This PR makes necessary changes to the Python code to allow us to run it in 1P situations. See cl/841700264 for context.

• a2ui_extension.py: A2UI_EXTENSION_URI is the message, but not in the request header (due to using Stubby instead of HTTP.)
• We need to use SecureRunner instead of Runner. The Runner is passed from the server code to the agent executor.
• File loading is also handled by the server code.
• Imports from local directories need dots (.) for 1P but can't be there for 3P
• The tools need parameters, or the Rizzcharts agent will say it cannot query for a specific time period (like Q1). Gemini claims this is a security feature of the SecureRunner.
• Other refactoring to enable the above.

[gemini-code-assist]

Code Review

This pull request refactors the Rizzcharts example agent to support running within internal Google infrastructure. The changes primarily involve updating how file resources are loaded and restructuring the agent and executor for better dependency injection. My review focuses on improving the robustness of the new file loading logic, ensuring documentation is complete, and making error handling more consistent. I've identified a bug in the file loading fallback mechanism and a style guide violation regarding missing tests for the new logic. I've also pointed out an incomplete docstring and an inconsistent error handling pattern.