LegacyDependencySubscriber discards other providers' data when one provider posts an update

[kfertitta]

When multiple IProjectDependenciesSubTreeProvider implementations fire DependenciesChanged, OnSubtreeProviderDependenciesChanged posts only the updated provider's snapshot to the broadcast block, discarding data from all other providers. This means whichever provider fires last wins — all other group nodes disappear from the Dependencies tree.

For example, given two providers (A and B):

1. Provider A fires → Post(providerStateA.Update(...)) → tree shows group A
2. Provider B fires → Post(providerStateB.Update(...)) → tree shows group B only, group A gone

Repro: Register two or more IProjectDependenciesSubTreeProvider exports from an extension. When both fire
DependenciesChanged, only the last provider's group nodes remain visible in the Dependencies tree.

Affected code: LegacyDependencySubscriber.Source.OnSubtreeProviderDependenciesChanged in src/Microsoft.VisualSt udio.ProjectSystem.Managed/ProjectSystem/Tree/Dependencies/Legacy/LegacyDependencySubscriber.cs

The current code posts the single provider's snapshot:

ProviderState providerState = GetProviderState(e.Provider);
Post(providerState.Update(e.Changes));

The following change resolved the issue for us — merging all provider states before posting:

ProviderState providerState = GetProviderState(e.Provider);
providerState.Update(e.Changes);

ImmutableDictionary<DependencyGroupType, ImmutableArray<IDependency>>.Builder merged =
    ImmutableDictionary.CreateBuilder<DependencyGroupType, ImmutableArray<IDependency>>();

foreach (ProviderState state in _stateByProvider.Values)
{
    foreach (KeyValuePair<DependencyGroupType, ImmutableArray<IDependency>> pair in state.Snapshot)
    {
        merged[pair.Key] = pair.Value;
    }
}

Post(merged.ToImmutable());

This also requires exposing the _snapshot field from ProviderState:

public ImmutableDictionary<DependencyGroupType, ImmutableArray<IDependency>> Snapshot => _snapshot;