fix: use []any instead of []string in loadartifactstool to fix structpb serialization

[brucearctor]

Description

Fixes #668

loadartifactstool.Run returns []string in its response map, which causes structpb.NewStruct to fail when the Vertex AI session service persists the function response event. structpb only supports []any, not typed slices like []string.

Changes

tool/loadartifactstool/load_artifacts_tool.go

1. Run(): Convert []string to []any before returning in the result map, ensuring compatibility with structpb.NewStruct.

2. processLoadArtifactsFunctionCall(): Replace the hard []string type assertion with a type switch that handles both []string (direct call) and []any (after a structpb round-trip where []string becomes []interface{}).

tool/loadartifactstool/load_artifacts_tool_test.go

• Updated existing test expectations from []string to []any to match the new return type.
• Added TestLoadArtifactsTool_Run_StructpbCompatibility: verifies that Run() output can be serialized via structpb.NewStruct (the core bug assertion).
• Added TestLoadArtifactsTool_ProcessRequest_Artifacts_LoadArtifactsFunctionCall_AnySlice: verifies ProcessRequest handles []any in the function response (simulating a round-trip).

Testing Plan

Unit Tests

go test ./tool/loadartifactstool/ -v -count=1

All 13 tests pass:

• 8 existing Run tests (updated expectations)
• 1 new StructpbCompatibility test
• 2 existing ProcessRequest tests
• 1 new ProcessRequest_AnySlice test
• 1 existing OtherFunctionCall test

Minimal Reproduction (from issue)

result := map[string]any{"artifact_names": []string{"a.pdf", "b.pdf"}}
_, err := structpb.NewStruct(result) // BEFORE: proto: invalid type: []string

// After fix, Run() returns []any:
result = map[string]any{"artifact_names": []any{"a.pdf", "b.pdf"}}
_, err = structpb.NewStruct(result)  // AFTER: <nil>