Skip missing static files instead of aborting the build

[official-burak]

Fixes #4048.

File.copy_file currently lets shutil.copyfile raise FileNotFoundError for a dangling symlink or a source that disappeared after discovery. That aborts mkdocs build / mkdocs serve even with strict: false.

This catches the missing source, logs a warning, and continues. is_modified() also treats an unreadable source as modified so a dirty rebuild takes the same warning path instead of crashing on os.path.getmtime.

Tests cover a dangling symlink, a deleted source file, and a dirty rebuild that already has a destination copy.

[official-burak]

test (3.11, ubuntu-latest) (and the other 3.10+/pypy jobs that actually created a hatch test env) failed on test_draft_docs_with_comments_from_user_guide, not on the new static-copy tests.

The static-copy change is unrelated. CI installs pathspec>=0.11.1; Python 3.9+ gets pathspec 1.x, which keeps leading whitespace (git-compatible). The user-guide fixture was indented like Python source, so *_unpublished.md never matched and draft pages were written into site_dir. Python 3.8 still installs pathspec 0.12.1, which stripped those spaces, which is why that job stayed green.

Pushed 67ac6c3: textwrap.dedent() the fixture. Locally, with pathspec 1.1.1:

python -m unittest mkdocs.tests.build_tests.BuildTests.test_draft_docs_with_comments_from_user_guide
mkdocs.tests.structure.file_tests.TestFiles.test_copy_file_dangling_symlink
mkdocs.tests.structure.file_tests.TestFiles.test_copy_file_missing_source
mkdocs.tests.structure.file_tests.TestFiles.test_copy_file_dangling_symlink_dirty
# OK

[official-burak]

Follow-up: test (3.11, ubuntu-latest) is green on 67ac6c3 (728 tests). Remaining red cells on this SHA fail before unittest:

• pypy ubuntu: hatch extra cryptography failed to build a wheel (maturin)
• pypy windows: hatch extra zstandard failed (No module named distutils.msvc9compiler)
• macOS 3.10: ValueError: Unknown environment collector: mkdocs while installing hatch

Those are hatch/native-dep install failures, not the static-copy change.