feat(lib): add files_from_zip helper for skill uploads

[lyr408]

Summary

Adds files_from_zip() and async_files_from_zip() helper functions to anthropic.lib, complementing the existing files_from_dir() helper.

Motivation

Currently, uploading a skill from a zip archive requires manually constructing file tuples:

skill = client.beta.skills.create(
    files=[("skill.zip", open("skill.zip", "rb"))],
    ...
)

With this change, users can use a dedicated helper that extracts individual files from the zip:

from anthropic.lib import files_from_zip

skill = client.beta.skills.create(
    files=files_from_zip("financial_analysis_skill.zip"),
    ...
)

Changes

• src/anthropic/lib/_files.py — Added files_from_zip(), async_files_from_zip(), and internal _read_zip() helper
  • Returns list[FileTypes] (2-tuples) consistent with files_from_dir()
  • Automatically skips directory entries and __MACOSX/ metadata
• src/anthropic/lib/__init__.py — Exported the new functions
• tests/lib/test_files.py — 11 tests covering sync/async, edge cases, and mixed tuple format validation
• examples/skills_from_zip.py — Usage example showing all three file upload patterns, including that the MIME type is optional in file tuples

All changes are in lib/, tests/, and examples/ — directories not affected by the code generator.