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GH-44256: [C++][FS][Azure] Fix edgecase where GetFileInfo incorrectly returns NotFound on flat namespace and Azurite (#44302)



### Rationale for this change
Fix a bug where `GetFileInfo` incorrectly returns `FileType::NotFound` on flat namespace and Azurite.

### What changes are included in this PR?
Fix by detecting the exact edgecase and doing an extra listing operation to disambiguate. 

### Are these changes tested?
Yes, updated automated test

### Are there any user-facing changes?
Only a bug fix.

* GitHub Issue: #44256

Authored-by: Thomas Newton <thomas.w.newton@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>