fix(skills): handle bytes in bundle update hash

[rylena]

Summary

• handle SkillBundle.files entries that are already bytes in bundle_content_hash()
• add regression coverage for byte-backed bundle files so skill update checks stop crashing

Verification

• scripts/run_tests.sh tests/tools/test_skills_hub.py -k 'bundle_content_hash or check_for_skill_updates'
• scripts/run_tests.sh tests/tools/test_skills_hub.py
• independent delegated code review: passed

Closes #19090
Related to #19073

[alt-glitch]

Duplicate of #13531, #3751, #2740, and #19081 — same one-line isinstance fix for bytes in bundle_content_hash(). Please consolidate.

[teknium1]

The same bytes fix landed via #19575 (salvage of @teknium1's #19328, which was itself the canonical consolidation of five concurrent PRs — including yours). The logic you proposed is identical; thanks for catching and fixing this bug. Both contributors who implemented the same fix get credit via the commit's AUTHOR_MAP and this acknowledgement. Closing as duplicate.