Problem

Three declarative workflow samples (Workflow_Declarative_FunctionTools, Workflow_Declarative_InvokeFunctionTool, Workflow_Declarative_ToolApproval) crash with KeyNotFoundException during the WORKFLOW: Restore step after a successful WORKFLOW: Yield.

Discovered via the nightly verify-samples run.

Root Cause

Bug in FileSystemJsonCheckpointStore.RetrieveCheckpointAsync (line 171):

string fileName = GetFileNameForCheckpoint(sessionId, key);  // relative filename only
string filePath = Path.Combine(this.Directory.FullName, fileName);  // full path

if (!this.CheckpointIndex.Contains(key) ||
    !File.Exists(fileName))   // BUG: resolves relative to CWD, not the store directory

File.Exists(fileName) resolves relative to the process current working directory — not the checkpoint store directory. When CWD ≠ store directory (as in CI), the file is never found, even though line 176 correctly uses filePath to open it.

Fix

• Changed File.Exists(fileName) → File.Exists(filePath) on line 171
• Added RetrieveCheckpointAsync_ShouldReturnPersistedDataAsync roundtrip unit test (existing tests only covered CreateCheckpointAsync)

Testing

• All 330 workflow unit tests pass (22 skipped as pre-existing flaky)
• New roundtrip test validates create → retrieve works correctly