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GH-40037: [C++][FS][Azure] Make attempted reads and writes against directories fail fast (#40119)



### Rationale for this change
Prevent confusion if a user attempts to read or write a directory. 

### What changes are included in this PR?
- Make `ObjectAppendStream::Flush` a noop if `ObjectAppendStream::Init` has not run successfully. This avoids an unhandled error when the destructor calls flush. 
- Check blob properties for directory marker metadata when initialising `ObjectInputFile` or `ObjectAppendStream`. 
- When initialising `ObjectAppendStream` call `GetFileInfo` if it is a flat namespace account.

### Are these changes tested?
Add new tests `DisallowReadingOrWritingDirectoryMarkers` and `DisallowCreatingFileAndDirectoryWithTheSameName` to cover the new fail fast behaviour. 
Also updated `WriteMetadata` to ensure that my changes to Flush didn't break setting metadata without calling `Write` on the stream. 

### Are there any user-facing changes?
Yes. Invalid read and write operations will now fail fast and gracefully. Previously could get into a confusing state where there were files and directories at the same path and there were some un-graceful failures. 

* Closes: #40037

Authored-by: Thomas Newton <thomas.w.newton@gmail.com>
Signed-off-by: Felipe Oliveira Carvalho <felipekde@gmail.com>