[Mgmt] Fix nested flattened collection constructor arguments

[Wei Hu (live1206)]

Description

Fixes constructor generation for nested collection properties that pass through both explicit property flattening and automatic safe flattening. When an outer public constructor exposes IEnumerable<T> but the nested constructor requires IList<T>, the generator now materializes the argument with .ToList().

The shared argument-building helper also consolidates the duplicated name-match and type-match conversion logic.

Adds an end-to-end TypeSpec regression fixture that reproduces the Cosmos DB constructor chain from #63263.

Fixes #63263

Validation

• pwsh eng/packages/http-client-csharp-mgmt/eng/scripts/Generate.ps1 -filter Mgmt-TypeSpec
• dotnet test eng/packages/http-client-csharp-mgmt/generator/Azure.Generator.Management/test/Azure.Generator.Mgmt.Tests.csproj --filter FullyQualifiedName~FlattenPropertyVisitorTests --no-restore

[azure-pipelines]

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[azure-pipelines]

Azure Pipelines:
Successfully started running 2 pipeline(s).
11 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[Copilot]

Copilot review overview

🟢 Approval recommended

The generator conversion is correctly scoped and the regenerated fixture verifies the previously incompatible constructor chain.

Review effort: Balanced
Findings: None

What changed in this PR

Fixes management generator constructor calls by materializing IEnumerable<T> as IList<T> when required by nested flattened models.

Changes:

• Consolidates flattened constructor argument conversion.
• Adds .ToList() for incompatible collection interfaces.
• Adds a TypeSpec regression fixture and regenerated outputs.

Package versioning and released-API compatibility checks are not applicable to this generator-only change.

File	Description
FlattenPropertyVisitor.cs	Adds shared argument conversion and list materialization.
nested-flattened-collection.tsp	Defines the regression scenario.
main.tsp	Imports the new fixture.
NestedFlattenedCollectionOperationsRestOperations.cs	Generated REST operation.
NestedFlattenedCollectionResource.Serialization.cs	Generated resource serialization.
NestedFlattenedCollectionResource.cs	Generated collection wrapper.
NestedFlattenedCollectionProperties.Serialization.cs	Generated properties serialization.
NestedFlattenedCollectionProperties.cs	Generated nested properties model.
NestedFlattenedCollectionItem.Serialization.cs	Generated item serialization.
NestedFlattenedCollectionItem.cs	Generated collection item.
NestedFlattenedCollectionContent.Serialization.cs	Generated request serialization.
NestedFlattenedCollectionContent.cs	Demonstrates the corrected .ToList() call.
AzureGeneratorMgmtTypeSpecTestsContext.cs	Registers generated models.
MgmtTypeSpecTestsModelFactory.cs	Adds model factory methods.
FooResource.cs	Exposes generated regression operations.

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