chore: import upstream snapshot with attribution

dotnet/samples/03-workflows/Declarative/AotCheckpointing/README.md

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+# AotCheckpointing sample
+
+Demonstrates JSON checkpointing of a declarative workflow under
+reflection-disabled `System.Text.Json` -- the same constraint imposed by
+**AOT / trim-aggressive deployments**.
+
+## What it shows
+
+- A 3-action declarative workflow
+  (`SetVariable` -> `InvokeAzureAgent` -> `SendActivity`) checkpointed
+  after every superstep.
+- The csproj sets
+  `<JsonSerializerIsReflectionEnabledByDefault>false</JsonSerializerIsReflectionEnabledByDefault>`.
+  Every JSON operation must resolve type info via a source-gen
+  `JsonSerializerContext` or fail with
+  `InvalidOperationException: No JSON type info is available for type 'X'`.
+- The experimental
+  [`DeclarativeWorkflowJsonOptions.Default`](../../../../src/Microsoft.Agents.AI.Workflows.Declarative/DeclarativeWorkflowJsonOptions.cs)
+  `JsonSerializerOptions` instance covers every declarative-package
+  type that flows through the checkpoint pipeline. Pass it to
+  `CheckpointManager.CreateJson`.
+- JSON round-trip is verified in two phases:
+  1. Run + drain -- every `[checkpoint x<n>]` line is a successful JSON **write**.
+  2. `ResumeStreamingAsync` on a fresh workflow instance -- a clean
+     return is the proof JSON **reads** round-trip too. The resumed run
+     is disposed immediately; without a pending external request it
+     would park in `WaitForInputAsync` indefinitely.
+
+`DeclarativeWorkflowJsonOptions` is marked
+`[Experimental("MAAI001")]`. Suppress that diagnostic in your csproj to
+use it.
+
+### Registering user-defined types
+
+For workflows whose inputs or custom `ActionExecutorResult.Result`
+payloads are user-defined, clone `Default` and append your own resolver:
+
+```csharp
+JsonSerializerOptions options = new(DeclarativeWorkflowJsonOptions.Default);
+options.TypeInfoResolverChain.Add(MyAppJsonContext.Default);
+options.MakeReadOnly();
+CheckpointManager manager = CheckpointManager.CreateJson(store, options);
+```
+
+## Run
+
+Prerequisites:
+
+- Azure Foundry project with a deployed model.
+- `az login`.
+- Configuration (user secrets or env):
+
+  | Setting | Description |
+  | --- | --- |
+  | `AZURE_AI_PROJECT_ENDPOINT` | Foundry project endpoint URL. |
+  | `AZURE_AI_MODEL_DEPLOYMENT_NAME` | Model deployment name. |
+
+  See the [parent README](../README.md) for the full walkthrough.
+
+```sh
+cd dotnet/samples/03-workflows/Declarative/AotCheckpointing
+dotnet run "Hello, my name is Ada."
+```
+
+Expected output:
+
+1. `[checkpoint x<n>]` lines after each superstep.
+2. The agent's streamed response.
+3. `ACTIVITY: [Sample] Workflow completed. ...`
+4. `WORKFLOW: Verifying read path by resuming from checkpoint <id>`
+5. `WORKFLOW: Checkpoint deserialized successfully`
+6. `WORKFLOW: Done!`
+
+The `chk-*/` checkpoint folder is deleted at the end.
+
+## Observe the failure mode
+
+Drop the options argument:
+
+```csharp
+CheckpointManager checkpointManager = CheckpointManager.CreateJson(store, DeclarativeWorkflowJsonOptions.Default);
+```
+
+becomes
+
+```csharp
+CheckpointManager checkpointManager = CheckpointManager.CreateJson(store);
+```
+
+Clean rebuild, then re-run. Expected on the first checkpoint commit:
+
+```
+System.InvalidOperationException: No JSON type info is available for type
+'Microsoft.Agents.AI.Workflows.Declarative.Kit.ActionExecutorResult'.
+```
+
+This is what `dotnet publish -p:PublishAot=true` would surface at runtime.
+
+## Notes
+
+- `PublishAot=true` is **not** set. The
+  `JsonSerializerIsReflectionEnabledByDefault=false` flag is the
+  minimum constraint that reproduces the AOT failure for JSON
+  checkpointing.
+- JSON code paths inside transitive dependencies (e.g. Foundry SDK)
+  that rely on reflection would also fail under this flag; those are
+  outside the workflow framework's responsibility.