.NET: fix: FileSystemJsonCheckpointStore does not flush to disk on Checkpoint creation

[lokitoth]

Description

Flushes the index file on checkpoint creation to ensure we get disk status current when control returns to user code after the checkpoint.

Fixes #3169

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

Fixes FileSystemJsonCheckpointStore so the index (index.jsonl) is flushed during checkpoint creation, ensuring the on-disk index is up to date when CreateCheckpointAsync returns (addresses #3169).

Changes:

• Flush index.jsonl after appending a checkpoint entry in CreateCheckpointAsync.
• Add a unit test verifying the index file has non-zero length before the store is disposed.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
dotnet/src/Microsoft.Agents.AI.Workflows/Checkpointing/FileSystemJsonCheckpointStore.cs	Flushes the index stream after writing a new checkpoint entry.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/FileSystemJsonCheckpointStoreTests.cs	Adds coverage to ensure index data is persisted prior to store disposal.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.