Python: [Feature]: Add an AgentFuse FunctionMiddleware sample

[MkaliezZ]

Description

Add a focused Python sample showing how an external AgentFuse RuntimeGuard can use the existing FunctionMiddleware boundary immediately before tool dispatch.

The sample should:

• preserve the host-provided call_id without fabricating one when it is missing;
• let allowed calls continue through the existing FunctionTool.invoke path;
• return a terminal non-execution result for policy blocks or guard evaluation failures;
• leave handler failures after an allow decision under Agent Framework ownership;
• preserve task cancellation;
• remain sample-only, with no framework API or production packaging changes.

This is a concrete external-policy example of the execution-boundary pattern discussed in #5366, but it does not attempt to close that broader issue.

Code Sample

Draft implementation: #7719

Language/SDK

Python