[Mgmt][CodeGen] Fix KeyNotFoundException in LRO generation when multiple resources share one ResourceData type

[Copilot]

Description

ResourceOperationMethodProvider.BuildLroHandling crashes with KeyNotFoundException when N≥2 ResourceClientProviders share a single ResourceData type and at least one owns an LRO (e.g. Billing role assignment / role definition MPG migration, where BillingRoleAssignmentData backs 6 distinct resources across different parent scopes).

Root cause is an asymmetry between populator and reader of OperationSourceDict:

• Populator (ManagementOutputLibrary.ProcessLroMethod) registers N entries keyed by each resourceProvider.Type.
• Reader (BuildLroHandling via TryGetResourceClientProvider) intentionally bails out (Count != 1) and falls through to the non-resource branch, which looks up the raw data type key — never registered → crash.

Changes

• ManagementOutputLibrary.ProcessLroMethod — when more than one resource provider matches the LRO return type, additionally register a fallback OperationSourceProvider keyed by the raw data type. This mirrors TryGetResourceClientProvider's "skip wrapping" intent and makes the dictionary symmetric with what BuildLroHandling reads in the non-wrap branch.

if (resourceProviders.Count > 0)
{
    foreach (var resourceProvider in resourceProviders)
    {
        operationSources.TryAdd(resourceProvider.Type, new OperationSourceProvider(resourceProvider));
    }

    // Mirror TryGetResourceClientProvider: when multiple resources share the same data type
    // the reader treats the response as a non-resource (raw data type).
    if (resourceProviders.Count > 1)
    {
        operationSources.TryAdd(returnCSharpType, new OperationSourceProvider(returnCSharpType));
    }
}

• OperationSourceProviderTests.Verify_MultipleResourcesSharingDataModel_GeneratesSeparateOperationSources — updated to assert the new fallback entry (SiteDataOperationSource) is present alongside the two resource-typed entries (count now 3).

This is Option A from the issue — the minimal fix matching the existing intent in TryGetResourceClientProvider. Option B (deterministic resource selection in the reader) is left for a future change if a friendlier ArmOperation<TResource> shape is desired.

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[Copilot]

Pull request overview

Fixes a crash in the management-plane generator’s LRO handling when multiple ResourceClientProviders share the same ResourceData type by making OperationSourceDict registration symmetric with the lookup behavior in ResourceOperationMethodProvider.BuildLroHandling.

Changes:

• Register an additional fallback OperationSourceProvider keyed by the raw data model type when multiple resources share that model type.
• Update the existing unit test to assert the new fallback operation source entry is generated.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
eng/packages/http-client-csharp-mgmt/generator/Azure.Generator.Management/src/ManagementOutputLibrary.cs	Adds a raw-data-type fallback entry in ProcessLroMethod when multiple resources match the LRO return model type, preventing OperationSourceDict lookup failures.
eng/packages/http-client-csharp-mgmt/generator/Azure.Generator.Management/test/Providers/OperationSourceProviderTests.cs	Updates assertions to expect the additional data-type-keyed fallback operation source alongside the per-resource entries.