GCS skill loaders ignore a skill.md manifest that the local and zip loaders accept #7842

🔴 Required Information

Describe the Bug:
A skill whose manifest is named skill.md loads from a local directory or a zip archive but is invisible when stored in GCS. _parse_skill_md, _list_skills_in_dir and the zip loader in skills/_utils.py all accept SKILL.md or skill.md. _list_skills_in_gcs_dir and _load_skill_from_gcs_dir only look for SKILL.md. Moving a skill that works locally into a bucket therefore makes it disappear from the listing, and loading it raises FileNotFoundError. The async wrappers call the same functions and behave the same way.

Steps to Reproduce:

1. Upload a skill directory with gs://my-bucket/skills/my-skill/skill.md (valid frontmatter, name: my-skill).
2. _list_skills_in_gcs_dir("my-bucket", "skills/").
3. _load_skill_from_gcs_dir("my-bucket", "my-skill", skills_base_path="skills/").

Expected Behavior:
The skill is listed and loads, as it does from a local directory.

Observed Behavior:
The listing is {}, and loading raises FileNotFoundError: SKILL.md not found at gs://my-bucket/skills/my-skill/SKILL.md.

Environment Details:

• ADK Library Version (pip show google-adk): 2.13.0 (also on main at 75fb788)
• Desktop OS: macOS
• Python Version (python -V): 3.11.16

Model Information:

• Are you using LiteLLM: No
• Which model is being used: N/A (skill loading)

🟡 Optional Information

Additional Context:
I verified this with the mocked google.cloud.storage client the existing GCS skill tests use; I don't have a bucket for a live run.

How often has this issue occurred?:

• Always (100%), for GCS skills whose manifest is skill.md