Remove generator-produced [Function] definitions for class-based tasks to avoid duplicates with native DF support

[Copilot]

Azure Functions Durable Extension PR #3229 added native support for class-based orchestrators, activities, and entities. The generator now produces duplicate [Function] definitions alongside the runtime's, causing potential conflicts.

Changes

• Generator: Remove [Function] attribute generation for class-based tasks (TaskOrchestrator<T>, TaskActivity<T>, TaskEntity<T>) when targeting Durable Functions
• Generator: Retain extension method generation (CallMyActivityAsync, ScheduleNewMyOrchestratorInstanceAsync) for type-safe invocation
• Generator: Skip generation entirely when only entities exist (no extension methods needed)
• Tests: Update 17 Azure Functions tests to expect extension methods only, not [Function] definitions
• Tests: Support null expected output for scenarios with no generated code

Before

// Generator produced both:
[Function(nameof(MyActivity))]
public static async Task<string> MyActivity([ActivityTrigger] int input, string instanceId, FunctionContext ctx) { ... }

public static Task<string> CallMyActivityAsync(this TaskOrchestrationContext ctx, int input) { ... }

After

// Generator produces only extension methods:
public static Task<string> CallMyActivityAsync(this TaskOrchestrationContext ctx, int input) { ... }

// Durable Functions runtime handles [Function] registration natively

Method-based activity triggers ([ActivityTrigger] on methods) continue generating extension methods as before.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Refactor Generator to handle updated Durable Functions class-based invocation</issue_title>
<issue_description>With this PR, Durable Functions is enabling class-based invocations. In doing so, the generator could now lead to duplicate function definitions. The generator should be updated to handle this new case.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Refactor Generator to handle updated Durable Functions class-based invocation #501

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