test(tools): add coverage for ForwardingArtifactService

[nuthalapativarun]

Link to Issue or Description of Change

2. Or, if no issue exists, describe the change:
Problem: ForwardingArtifactService (used by AgentTool to forward artifact operations from a sub-agent's ToolContext back to the root invocation context) has no unit test coverage. In particular, list_artifact_versions and get_artifact_version are unimplemented stubs that raise NotImplementedError, and the delegation logic in save_artifact, load_artifact, list_artifact_keys, delete_artifact, and list_versions is untested.
Solution: Add tests/unittests/tools/test_forwarding_artifact_service.py with unit tests covering:

• Delegation of save_artifact, load_artifact, and list_artifact_keys to the wrapped ToolContext.
• Delegation of delete_artifact and list_versions to the root invocation context's artifact_service, including the ValueError raised when no root artifact service is configured.
• NotImplementedError raised by both unimplemented stubs (list_artifact_versions, get_artifact_version).

This is a test-only change with no behavior modifications.

Testing Plan

Unit Tests:

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

tests/unittests/tools/test_forwarding_artifact_service.py::test_save_artifact_delegates_to_tool_context PASSED
tests/unittests/tools/test_forwarding_artifact_service.py::test_load_artifact_delegates_to_tool_context PASSED
tests/unittests/tools/test_forwarding_artifact_service.py::test_list_artifact_keys_delegates_to_tool_context PASSED
tests/unittests/tools/test_forwarding_artifact_service.py::test_delete_artifact_delegates_to_invocation_context_service PASSED
tests/unittests/tools/test_forwarding_artifact_service.py::test_delete_artifact_raises_when_no_root_artifact_service PASSED
tests/unittests/tools/test_forwarding_artifact_service.py::test_list_versions_delegates_to_invocation_context_service PASSED
tests/unittests/tools/test_forwarding_artifact_service.py::test_list_versions_raises_when_no_root_artifact_service PASSED
tests/unittests/tools/test_forwarding_artifact_service.py::test_list_artifact_versions_raises_not_implemented PASSED
tests/unittests/tools/test_forwarding_artifact_service.py::test_get_artifact_version_raises_not_implemented PASSED

9 passed in 1.68s

Manual End-to-End (E2E) Tests:
N/A — test-only addition, no runtime behavior change.

Checklist

• Read CONTRIBUTING.md
• Self-review performed
• Comments added for hard-to-understand areas
• Tests added proving fix/feature works
• New and existing unit tests pass locally
• Manually tested end-to-end
• Dependent changes merged/published downstream

Additional context

No associated issue — this is a pure test-coverage addition with no behavior change, per CONTRIBUTING.md's allowance for small, focused PRs.