chore(librarian): resolve issue where generation failed for certain packages

[parthea]

This PR fixes an issue where generation fails for libraries when /input/packages/google-cloud-<api> does not exist.

Traceback (most recent call last):
  File "/app/./cli.py", line 620, in handle_generate
    _copy_files_needed_for_post_processing(output, input, library_id, is_mono_repo)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/./cli.py", line 372, in _copy_files_needed_for_post_processing
    shutil.copytree(
    ~~~~~~~~~~~~~~~^
        source_dir,
        ^^^^^^^^^^^
    ...<2 lines>...
        ignore=shutil.ignore_patterns("client-post-processing"),
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/usr/local/lib/python3.14/shutil.py", line 652, in copytree
    with os.scandir(src) as itr:
         ~~~~~~~~~~^^^^^
FileNotFoundError: [Errno 2] No such file or directory: '/input/packages/google-cloud-hypercomputecluster'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/app/./cli.py", line 1582, in <module>
    args.func(
    ~~~~~~~~~^
        librarian=args.librarian,
        ^^^^^^^^^^^^^^^^^^^^^^^^^
    ...<2 lines>...
        input=args.input,
        ^^^^^^^^^^^^^^^^^
    )
    ^
  File "/app/./cli.py", line 628, in handle_generate
    raise ValueError("Generation failed.") from e
ValueError: Generation failed.