Fix fastloop initialization and add regression test

[luke-lombardi]

Fixes infer_application_path to correctly determine the user's application module, preventing Hypercorn NoAppError.

Commit 38148e9 introduced a bug where infer_application_path would incorrectly fall back to app_instance (the FastLoop instance itself) if no app attribute was found. This caused app_instance.__module__ to return "fastloop.fastloop", leading Hypercorn to search for fastloop.fastloop:app, which does not exist, resulting in a NoAppError. The fix ensures that if no app attribute is present, the function correctly proceeds to argv-based inference to find the user's module. Regression tests have been added to prevent future regressions.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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