Added a warning for non exportable types in export-dir

[varundeepsaini]

Fixes: #4081

Changes

Skip non-exportable workspace objects (like MLFLOW_EXPERIMENT) during workspace export-dir instead of failing the entire export. Warnings are collected and displayed at the end.

Why

Currently, export-dir fails when a directory contains an MLflow experiment because experiments cannot be exported via the workspace export API. This makes it impossible to export any folder containing an experiment.

This change skips non-exportable object types (LIBRARY, DASHBOARD, REPO, MLFLOW_EXPERIMENT) and shows warnings at the end:

Exporting files from /my/folder

Warnings:
  - /my/folder/my-experiment (skipped; cannot export MLFLOW_EXPERIMENT)

Export complete

Note: Ideally, ObjectTypeMlflowExperiment should be added to databricks-sdk-go as a constant. Currently using a string literal as a workaround.

Tests

• Added acceptance test cmd/workspace/export-dir-skip-experiments
• Updated cmd/workspace/export-dir-file-size-limit output format
• Verified manually against a real workspace with an MLflow experiment

[github-actions]

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Inputs:

• PR number: 4101
• Commit SHA: cb936775a367b9e73063cd539b5a3e9ca85dd583

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