feat(artifacts): add LocalFileArtifactService for local file system storage

[eduramirezh]

This adds a new LocalFileArtifactService implementation that stores artifacts
on the local file system instead of cloud storage or memory, providing developers
with a local persistence option for development, testing, and deployment scenarios.

Key features:

• File system storage with configurable base path (defaults to ./adk_artifacts)
• Maintains consistent directory structure with user/session namespacing
• Stores MIME type metadata in separate .metadata.json files
• Full async support using asyncio.to_thread() for non-blocking I/O
• Proper error handling for corrupted metadata and missing files
• Sequential integer versioning starting from version 0

Changes:

• Add LocalFileArtifactService class implementing BaseArtifactService interface
• Add comprehensive standalone unit tests for all functionality
• Integrate with existing parametrized tests for consistency validation
• Export new service in artifacts/init.py
• Fix minor typo in CONTRIBUTING.md

Testing Plan

Unit Tests

All unit tests pass with comprehensive coverage:

Standalone Tests (24 tests):

• test_local_file_artifact_service.py covers:
  • Basic CRUD operations (save/load/delete)
  • Version management and specific version loading
  • User-namespaced vs session-scoped file handling
  • MIME type preservation across different content types
  • Directory structure validation
  • Error handling (corrupted metadata, missing files)
  • Edge cases (nonexistent versions, empty lists)

Integration Tests (24 additional tests):

• Extended existing parametrized tests in test_artifact_service.py
• Added LOCAL_FILE variant to all existing test cases
• Validates consistency with InMemoryArtifactService and GcsArtifactService

Test Results:

========================= 48 passed, 1 warning in 1.75s =========================

Note: The 1 warning is a pre-existing Pydantic warning unrelated to this change.

Code Quality:

• pylint: 10.00/10 (perfect score)
• mypy: Success, no type errors
• Formatted with pyink and isort per repository standards

Manual E2E Testing

The LocalFileArtifactService can be manually tested by:

1. Basic Usage:

   from google.adk.artifacts import LocalFileArtifactService
   service = LocalFileArtifactService(base_path="./test_artifacts")

2. Directory Structure Verification:
   After saving artifacts, verify correct file system layout:

   • Regular files: ./test_artifacts/{app}/{user}/{session}/{filename}/{version}
   • User files: ./test_artifacts/{app}/{user}/user/{filename}/{version}
   • Metadata files: {path}.metadata.json alongside each artifact

3. Integration Testing:
   Can be used as drop-in replacement for other artifact services in any
   existing ADK application or workflow.

This implementation maintains full API compatibility with existing artifact
services while providing local file system persistence capabilities.

[adk-bot]

Response from ADK Triaging Agent

Hello @eduramirezh, thank you for creating this PR!

This PR is a new feature, could you please associate a GitHub issue with this PR? If there is no existing issue, could you please create one?

This information will help reviewers to review your PR more efficiently. Thanks!

[eduramirezh]

Related issue: #2410