Fix MPG generator to emit model factory methods for models with flattened read-only properties

[Copilot]

Description

The MPG generator was not emitting model factory methods for models with flattened read-only properties. Models like QuotaAllocationRequestBase that have internal nested properties (Properties.Name) with read-only flattened accessors (Value, LocalizedValue) were incorrectly excluded from model factory generation, requiring custom workaround code.

Root cause: Model factory eligibility was determined during ModelFactoryVisitor.VisitType() before FlattenPropertyVisitor had processed all models. Models were evaluated pre-flattening and incorrectly excluded.

Changes made:

• ManagementOutputLibrary.cs: Exposed EnsureModelTypesBuilt() to allow explicit invocation after all model transformations
• ModelFactoryVisitor.cs: Deferred filtering to PostVisitType() and explicitly calls EnsureModelTypesBuilt() to ensure eligibility is evaluated after flattening completes

Example impact:

// Previously missing, now auto-generated:
public static QuotaAllocationRequestBase QuotaAllocationRequestBase(
    long? limit = null, 
    string value = null,           // flattened from Properties.Name.Value
    string localizedValue = null,  // flattened from Properties.Name.LocalizedValue
    string region = null)
{
    return new QuotaAllocationRequestBase(
        new QuotaAllocationRequestBaseProperties(
            limit, 
            new QuotaAllocationRequestBasePropertiesName(value, localizedValue, null), 
            region, 
            null), 
        null);
}

SDK teams can now remove custom model factory workarounds after regeneration.

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Original prompt

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This section details on the original issue you should resolve

<issue_title>Method missing in ModelFactory with new MPG generator</issue_title>
<issue_description>While generate SDK for Azure.ResourceManager.Quota, QuotaAllocationRequestBase is missing.
Added it back with custom code for now in

azure-sdk-for-net/sdk/quota/Azure.ResourceManager.Quota/src/Custom/ArmQuotaModelFactory.cs

Lines 96 to 106 in 30fa6a7

	// Will remove this custom code while https://github.com/Azure/azure-sdk-for-net/issues/54298 is fixed.
	/// <summary> Initializes a new instance of <see cref="Models.QuotaAllocationRequestBase"/>. </summary>
	/// <param name="limit"> The new quota limit for the subscription. The incremental quota will be allocated from pre-approved group quota. </param>
	/// <param name="value"> Resource name. </param>
	/// <param name="localizedValue"> Resource display name. </param>
	/// <param name="region"> The location for which the subscription is allocated. </param>
	/// <returns> A new <see cref="Models.QuotaAllocationRequestBase"/> instance for mocking. </returns>
	public static QuotaAllocationRequestBase QuotaAllocationRequestBase(long? limit = null, string value = null, string localizedValue = null, string region = null)
	{
	return new QuotaAllocationRequestBase(new QuotaAllocationRequestBaseProperties(limit, new QuotaAllocationRequestBasePropertiesName(value, localizedValue, additionalBinaryDataProperties: null), region, additionalBinaryDataProperties: null), additionalBinaryDataProperties: null);
	}

</issue_description>

<agent_instructions>Investigate why the method is missing from generated model factory. The goal is to fix MPG generator to generate it and remove the custom code.</agent_instructions>

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

• Fixes Method missing in ModelFactory with new MPG generator #54298

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