Tag deletion call-outs create noise in stack what-if results; deployment stacks should cover tags

**Is your feature request related to a problem? Please describe.**

Deployment stacks don't currently manage tags on the resources they manage. As a result, what-if output against a stack produces a lot of noise from tag deletion call-outs: every tag that exists on the deployed resource but isn't expressed in the template shows up as a pending removal, even though the stack isn't actually going to reconcile tags.

For an estate where tagging is applied by policy, inherited from the resource group, or set out-of-band by another tool, this makes the what-if result much harder to read and erodes trust in the preview. The signal that matters (resources being detached or deleted, deny setting changes) gets buried under tag diffs that will never be acted on.

**Describe the solution you'd like**

Two things, ideally in this order:

1. **Suppress the noise now.** Since tags aren't covered by deployment stacks, tag-only differences shouldn't be reported as changes in stack what-if results. Fold them into the existing noise reduction filter so they don't appear as deletions.
2. **Cover tags properly.** Longer term, it would be ideal if deployment stacks managed tags the same way they manage other resource properties, so the what-if preview reflects a change the stack will actually make.

**Describe alternatives you've considered**

Filtering the output client-side after the fact. That works, but it requires every consumer to reimplement the same filter, and it defeats the purpose of having noise reduction built into the service.

**Additional context**

Raised by a customer during early adoption of stacks what-if after GA. Related but distinct from #249, which is about tags not being exposed on the `Microsoft.Resources/deploymentStacksWhatIfResults` resource itself in the PowerShell cmdlets; this issue is about tags on the *managed resources* and the noise their diffs create in the result.

Start by tracing the deployment stacks what-if noise reduction filter and how tag differences on managed resources are generated. Compare tag-only deletions with resource detachments, deletions, and deny-setting changes. Done means tag-only differences are omitted from stack what-if results without hiding changes the stack will act on.

Written by the indexing model from the issue text.