Fixes #6332

Running adk api_server --a2a (or get_fast_api_app(..., a2a=True)) silently mounts no A2A routes. The gemini_enterprise_app_name block near the end of get_fast_api_app has a redundant function-local import json. Because of that local import, json is a local variable for the entire function, so the earlier json.load(f) in the A2A discovery loop raises UnboundLocalError: cannot access local variable 'json'. That exception is caught by the loop's except Exception, logged as "Failed to setup A2A agent", and swallowed, so every A2A agent silently fails to register and no /a2a/... routes are added.

json is already imported at module scope, so this removes the redundant local import. (The sibling import inspect in that block is kept: inspect is not imported at module scope.)

The existing A2A test only checks /list-apps, so it passed despite the broken route mounting. The added regression test drives get_fast_api_app(a2a=True) over a temp agent directory and asserts the A2A application is actually constructed (i.e. execution gets past the json.load call). It fails on main with the UnboundLocalError path and passes with this change; the existing A2A tests continue to pass.