fix(artifacts): Support nested API names

[spectaclehong]

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Link to Issue or Description of Change

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

• Fixes: Artifact API cannot load artifacts with path separators in names #6050

Problem:

Artifact services can save and list logical artifact names containing /, such as reports/summary.txt. The REST API also accepts these names through POST /artifacts because the filename is provided in the request body.

However, read/version/metadata/delete routes treated artifact_name as a single path segment. That made nested artifact names visible in the list endpoint but unreachable through the path-based endpoints.

Solution:

Update artifact read/version/metadata/delete routes to use FastAPI's {artifact_name:path} converter so nested logical artifact names can round-trip through the REST API.

Because {artifact_name:path} is catch-all, the version-specific routes are registered before the general artifact load route. No response schema changes are intended: endpoints that returned types.Part still return types.Part, and metadata endpoints still return ArtifactVersion / list[ArtifactVersion].

Also update artifact URI parsing so artifact references can resolve nested filenames such as folder/file.txt.

Testing Plan

Unit Tests:

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

Targeted artifact API tests pass locally:

uv run pytest tests/unittests/cli/test_fast_api.py -k artifact -q

Result:

9 passed, 72 deselected

Artifact service and URI tests pass locally:

uv run pytest tests/unittests/artifacts/test_artifact_util.py tests/unittests/artifacts/test_artifact_service.py -q

Result:

77 passed

Python 3.10 tox unit suite passes locally:

uv tool run --from tox --with tox-uv tox -e py310

Result:

7045 passed, 21 skipped, 31 xfailed, 9 xpassed

Pre-commit hooks pass for changed files:

uv run pre-commit run --files src/google/adk/cli/api_server.py src/google/adk/artifacts/artifact_util.py tests/unittests/cli/test_fast_api.py tests/unittests/artifacts/test_artifact_util.py tests/unittests/artifacts/test_artifact_service.py

Manual End-to-End (E2E) Tests:

Validated a local REST API round-trip using the ADK API server route stack with InMemoryArtifactService:

• save reports/summary.txt
• list artifacts
• load with raw slash path
• load with encoded slash path
• list versions
• load version 0
• load version metadata
• delete with encoded slash path

All requests returned 200.

Checklist

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

Additional context

No dependent changes are required.