1. Link to an existing issue (if applicable):

• Closes: Add async counterparts for loading and listing skills #6057

Problem:
Currently, all local and GCS skill loading and listing functions (load_skill_from_dir, load_skill_from_gcs_dir, list_skills_in_dir, list_skills_in_gcs_dir) are strictly synchronous and perform blocking disk or network I/O.

While the SkillRegistry abstract interface uses async methods, implementing a registry that loads local/GCS skills or using these loading utilities inside an asynchronous web server (like FastAPI) or async runners causes the main event loop to block, degrading performance and responsiveness

Solution:

Add asynchronous counterparts for all 4 core skill loading/listing functions in the google.adk.skills package, preserving the existing synchronous functions for backward compatibility:

load_skill_from_dir_async(skill_dir)
load_skill_from_gcs_dir_async(bucket_name, skill_id, ...)
list_skills_in_dir_async(skills_base_path)
list_skills_in_gcs_dir_async(bucket_name, ...)

These functions should utilize Python's built-in asyncio.to_thread to offload blocking I/O operations to a background thread pool, keeping the async event loop completely non-blocking.

Testing Plan

This change was verified using local unit tests. Four new asynchronous test cases were added to tests/unittests/skills/test__utils.py to validate the behavior of the new async APIs under different scenarios (local directories and Google Cloud Storage).

The following new unit tests were added:

test_load_skill_from_dir_async: Verifies that a skill (including instructions, frontmatter, and assets/references) is correctly loaded asynchronously from a local directory.
test_list_skills_in_dir_async: Verifies that skills in a parent directory are discovered asynchronously and their lightweight metadata is returned correctly.
test_load_skill_from_gcs_dir_async: Verifies that a skill is loaded asynchronously from Google Cloud Storage (GCS), with mock storage client interactions.
test_list_skills_in_gcs_dir_async: Verifies that skills stored in GCS are listed and discovered asynchronously using mock GCS directory prefixes.

Unit Tests:

• [X ] I have added or updated unit tests for my change.
• [ X] All unit tests pass locally.

Summary of passed pytest results.

============================= test session starts ==============================
platform linux -- Python 3.11.9, pytest-9.0.3, pluggy-1.6.0
rootdir: /usr/local/google/home/godwinpaul/Documents/source/adk-python
configfile: pyproject.toml
plugins: langsmith-0.8.11, asyncio-1.4.0, anyio-4.13.0, xdist-3.8.0, mock-3.15.1
asyncio: mode=Mode.AUTO, debug=False, asyncio_default_fixture_loop_scope=function, asyncio_default_test_loop_scope=function
collecting ...
collecting 21 items
collected 21 items

tests/unittests/skills/test__utils.py ..................... [100%]

======================== 21 passed, 4 warnings in 2.74s ========================

Manual End-to-End (E2E) Tests:

1. Setup a Mock Skill Directory called temp_skills with a dummy skill (my_dummy_skill)
2. Create a Test Script

import asyncio
import pathlib
from google.adk.skills import load_skill_from_dir_async

async def main():
    skill_path = pathlib.Path("temp_skills/my-dummy-skill")
    
    print(" [1/3] Loading skill asynchronously...")
    skill = await load_skill_from_dir_async(skill_path)
    
    print(" [2/3] Skill loaded successfully!")
    print(f"  • Name: {skill.name}")
    print(f"  • Description: {skill.description}")
    print(f"  • Instructions: {skill.instructions.strip()}")
    
    print(" [3/3] Accessing async-loaded resources...")
    ref_content = skill.resources.get_reference("test_reference.txt")
    print(f"  • Reference content: '{ref_content.strip()}'")

if __name__ == "__main__":
    asyncio.run(main())

3. Run the Script to see the skill loaded correctly.

Checklist

• [ X] I have read the CONTRIBUTING.md document.
• [X ] I have performed a self-review of my own code.
• [X ] I have commented my code, particularly in hard-to-understand areas.
• [ X] I have added tests that prove my fix is effective or that my feature works.
• [ X] New and existing unit tests pass locally with my changes.
• [ X] I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.