.NET: Add Workflow Builder Specialized Edge tests

[Copilot]

Motivation and Context

The WorkflowBuilderExtensions.ForwardMessage and ForwardExcept methods were materializing an intermediate List<ExecutorBinding> on every call, even when only a single target was passed. This is unnecessary allocation that can be avoided when the enumerable's count can be determined without enumeration. Additionally, the single-target optimization path was missing for ForwardMessage. The SwitchBuilder.AddCase and WithDefault methods also lacked null-validation for individual executor elements within the collection parameters. Finally, there was no unit test coverage for the edge types produced by the workflow builder extension methods.

Description

Performance Optimization (WorkflowBuilderExtensions):

• Refactored ForwardMessage<TMessage> and ForwardExcept<TMessage> to avoid unnecessary list materialization
• Added TFM-specialized count detection:
  • .NET (core): Uses TryGetNonEnumeratedCount for O(1) count detection
  • netstandard2.0/net472: Uses ICollection<ExecutorBinding> { Count: 1 } pattern matching
• Single target fast path creates a direct edge via AddEdge
• Multiple targets lazily validate via targets.Select(ValidateTarget) passed to AddSwitch

Null-Validation (SwitchBuilder and WorkflowBuilderExtensions):

• Added Throw.IfNull for builder and source in ForwardMessage and ForwardExcept IEnumerable overloads
• Added Throw.IfNull for executors parameter in AddChain
• Added per-element Throw.IfNull(executor, $"{nameof(executors)}[{executorIndex++}]") in SwitchBuilder.AddCase and WithDefault foreach loops with inline index tracking
• Added per-element Throw.IfNull(target, nameof(targets)) validation in ForwardMessage/ForwardExcept via ValidateTarget local function
• Added Throw.IfNull(target) in single-target ForwardMessage and ForwardExcept overloads

New Unit Tests (WorkflowBuilderSmokeTests):
Added comprehensive smoke tests for workflow edge type construction and argument validation:

• ForwardMessage_WithSingleTarget_CreatesDirectEdge — verifies direct edge and type predicate behavior
• ForwardMessage_WithMultipleTargets_CreatesFanOutEdge — verifies fan-out edge with condition
• ForwardExcept_WithSingleTarget_CreatesDirectEdge — verifies exclusion predicate
• ForwardExcept_WithMultipleTargets_CreatesFanOutEdge — verifies fan-out exclusion
• AddChain_CreatesSequentialDirectEdges — verifies sequential direct edge chain
• AddChain_WhenExecutorRepeats_Throws — verifies cycle detection
• AddExternalCall_CreatesRequestPortAndRoundTripEdges — verifies port and round-trip edges
• AddSwitch_CreatesFanOutEdgeWithCasesAndDefault — verifies switch with cases and default
• ForwardMessage_InvalidArguments_Throw — null/empty argument validation
• ForwardExcept_InvalidArguments_Throw — null/empty argument validation
• AddChain_InvalidArguments_Throw — null/duplicate argument validation
• AddExternalCall_InvalidArguments_Throw — null argument validation
• AddSwitch_InvalidArguments_Throw — null/empty argument validation
• SwitchBuilder_InvalidArguments_Throw — null element-in-collection validation with indexed parameter names

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.