file_copy process plugin does not validate downloaded file integrity, creating empty files on network failure

Problem/Motivation

The file_copy migrate process plugin does not validate the
integrity of a file after copying or downloading it. When the source is a
remote URL and a network failure occurs (e.g. timeout, connection reset) during
the download, the plugin creates an empty (zero-byte) file at the destination
path and registers it in file_managed with the expected filesize
from the source, rather than the actual size on disk.

This leads to the following issues:

• A file entity is persisted in the database with an incorrect
  filesize value, making it impossible to detect the corruption
  via the API.
• Modules relying on the file dimensions or content (e.g.
  responsive_image) throw unrecoverable exceptions such as
  LogicException: Could not determine image width, which can
  cause the entire page to return a 500 error.
• The migration reports the row as successfully processed, hiding the failure
  from operators.

Steps to reproduce

1. Set up a migration using the file_copy process plugin with a
   remote URL as source (e.g. via data_fetcher_plugin: file).
2. Simulate a network failure or timeout during the migration (e.g. by
   interrupting connectivity to the remote host mid-transfer).
3. Observe that a zero-byte file is created at the destination path, and a
   file entity is created in the database pointing to it.

Proposed resolution

After the copy/download operation, file_copy should verify that
the destination file exists and has a non-zero size. If the file is empty, it
should be deleted and a MigrateSkipRowException should be thrown
so the row is flagged as failed rather than silently processed.

Remaining tasks

• Patch file_copy to add post-copy integrity validation.
• Add a test covering the zero-byte file scenario.
• Update documentation.

User interface changes

None.

Introduced terminology

None.

API changes

FileCopy::transform() will throw a
MigrateSkipRowException when the destination file is empty after
the copy/download operation.

Data model changes

None.

Release notes snippet

The file_copy migrate process plugin now validates that the
destination file is non-empty after a copy or download operation. If an empty
file is detected (e.g. due to a network failure), the file is deleted and the
migration row is skipped with an explicit error, preventing corrupt file
entities from being created silently.