fix(cli): remove shadowing local json import breaking A2A setup

[AnayGarodia]

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.

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[AnayGarodia]

@googlebot I signed it!

[AnayGarodia]

@googlebot I signed it!

[AnayGarodia]

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[rohityan]

Hi @AnayGarodia , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Your PR has been received by the team and is currently under review. We will provide feedback as soon as we have an update to share.

[rohityan]

Hi @wyf7107 , can you please review this. LGTM

[AnayGarodia]

Rebased onto main to clear the conflict.

Heads-up from the rebase: the underlying UnboundLocalError is already resolved on main — commit 38587d7 (the a2a-sdk compatibility-shim refactor) removed the function-local import json as a side effect. So this PR now carries just the regression test that locks the behavior in. That fix landed without a test, and a future function-local import json would silently reintroduce the shadowing, so the guard still earns its place.

The test is skipped on the a2a-sdk v1 path (_compat.IS_A2A_V1), matching the sibling A2A tests that mock A2AStarletteApplication. Happy to retitle to test(cli): ... if you'd prefer given the fix itself is now upstream.