.NET: Declarative workflows - Gracefully handle agent scenarios when no response is returned

[peibekwe]

Motivation and Context

Declarative workflow runs that include InvokeAzureAgent actions could crash with:

 Unhandled workflow failure - #<action_id> (InvokeAzureAgent)
 → System.InvalidOperationException: Sequence contains no elements

The crash is deterministic whenever an invoked agent returns an AgentResponse with zero Messages (e.g., a tool-only
turn where the model emits remote_function_call / mcp_call items without a final assistant text message,
content-filtered responses, or streams that close before the terminal assistant message). When that happens, the
executor's unguarded .Last on the empty collection throws, DeclarativeActionExecutor wraps it as Unhandled
workflow failure - #{id} ({ModelType}), and the entire run terminates mid-pipeline — subsequent actions never
execute, so downstream side effects never happen.

Fixes #5375

Description

Replace unguarded .Last / .Single on response-message collections with defensive access that degrades gracefully.

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

Improves resilience of declarative workflow executors when an invoked agent returns an AgentResponse/ExternalInputResponse with zero Messages, avoiding deterministic crashes caused by unguarded .Last() / .Single() calls.

Changes:

• Replace .Last() usages with LastOrDefault() + null-guards across multiple executors to avoid “Sequence contains no elements”.
• Skip JSON response-object assignment when there is no final message text to parse.
• Add unit tests covering empty-message responses for RequestExternalInputExecutor.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/ObjectModel/RequestExternalInputExecutor.cs	Avoids .Last() crash when external input contains no messages.
dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/ObjectModel/QuestionExecutor.cs	Adds defensive handling around selecting the “input” message from an extracted response.
dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/ObjectModel/InvokeAzureAgentExecutor.cs	Avoids .Last() crash on resume and guards JSON parsing when last message is missing/empty.
dotnet/src/Microsoft.Agents.AI.Workflows.Declarative.Foundry/AzureAgentProvider.cs	Replaces .Single() with explicit cardinality check + clearer exception messages.
dotnet/tests/Microsoft.Agents.AI.Workflows.Declarative.UnitTests/ObjectModel/RequestExternalInputExecutorTest.cs	Adds coverage for empty-message responses (with and without workflow conversation).