Validate stereo fusion output path before running fusion

[behnamasadi]

RunStereoFuserImpl validated workspace_format, input_type and output_type up front, but never checked output_path until the results were written by WriteBinaryPlyPoints / Reconstruction::Write* after fusion completed. On large datasets fusion can run for hours, so a misspecified output_path (e.g. an existing directory when output_type is PLY, or a non-existent parent directory) aborted the whole run at the very end with a deferred THROW_CHECK_FILE_OPEN, discarding all the fusion work (issue #3117).

Add an early, output-type-aware validation of output_path in RunStereoFuserImpl before constructing the fuser:

• PLY writes a single file, so reject an existing directory and require the parent directory to exist.
• BIN/TXT write a directory of files, so require it to already exist (matching the THROW_CHECK_DIR_EXISTS in Reconstruction::Write*).

Also add a ply_test case asserting that WriteTextPlyPoints / WriteBinaryPlyPoints on a directory path fail cleanly, documenting the underlying deferred-failure behavior the early check guards against.