Microsoft.MarketplaceOrdering: cannot cancel or delete Anthropic Claude (Foundry) agreement after publisher withdraws plan from catalog

## Summary

After deploying Claude on Azure AI Foundry and then tearing down the deployment, the underlying `Microsoft.MarketplaceOrdering` agreement for the Anthropic plan is left `Active` and **cannot be cancelled or deleted** by the customer. Every mutating call returns `400 BadRequest`:

```
"The Publisher: 'anthropic' does not make available Offer: 'anthropic-claude-sonnet-4-6-offer',
Plan: 'anthropic-claude-sonnet-4-6-test-plan' in your Subscription/Azure account's region: 'Neutral'."
```

The plan was withdrawn from the Marketplace catalog by the publisher. The RP validates `cancel` / `delete` against the live catalog, so once the plan is gone the customer's agreement is stranded indefinitely.

## API

`Microsoft.MarketplaceOrdering` — `MarketplaceAgreements_Cancel` / `_Delete` / `_Get`:

```
POST /subscriptions/{sub}/providers/Microsoft.MarketplaceOrdering/agreements/{publisher}/offers/{offer}/plans/{plan}/cancel?api-version=2021-01-01
DELETE /subscriptions/{sub}/providers/Microsoft.MarketplaceOrdering/agreements/{publisher}/offers/{offer}/plans/{plan}?api-version=2021-01-01
```

Spec: `specification/marketplaceordering/`. Reproduces on `2015-06-01` too.

Note: Foundry Claude deployments auto-sign this agreement via `Microsoft.CognitiveServices` during the first model deployment — customers often don't realize they signed it until they try to clean up.

## Repro

1. Deploy Claude on Foundry (`Microsoft.CognitiveServices/accounts/.../deployments`), which auto-signs the underlying `Microsoft.MarketplaceOrdering` agreement.
2. Tear down the Foundry account.
3. Publisher later rotates / withdraws the plan from the catalog.
4. `az rest --method post --url .../cancel?api-version=2021-01-01` → `400 BadRequest` (above).
5. `DELETE` on the agreement → same error. Portal Cancel button → same error.
6. Collection `GET` still returns the agreement as `Active`; only the single-resource `GET` and all mutations fail.

Live correlation IDs (2026-05-28, plan `anthropic-claude-sonnet-4-6-test-plan`): `0fa3ab6f-3087-4557-9b30-a5791abe988d`, `c393bc9b-7ed1-429a-a056-699d6c9c2e2d`, `e4c81638-ccab-45b3-b6fb-66a684bb2451`. Another plan on the same offer (`-plan-new`) cancelled normally moments earlier — isolating the failure to catalog withdrawal.

## Expected

`cancel` and `delete` on an agreement the customer owns should succeed unconditionally. The agreement record is subscription-scoped data; its lifecycle should not depend on whether the publisher still lists the plan.

## Impact

- Customers cannot clean up Claude/Foundry marketplace agreements after teardown — `azd down` / CI teardown / compliance scans can never reach zero active agreements.
- Only workaround is a manual support ticket per stranded agreement.
- Will recur for every Anthropic / other AI-MaaS plan-id rotation; scales with Foundry adoption.

## Suggested fix

In `Microsoft.MarketplaceOrdering`, skip the publisher-catalog lookup on `cancel`, `delete`, and single-resource `get`. The agreement either exists in the customer's subscription or it doesn't.

Same component / service team as #12838.

Start in specification/marketplaceordering/ by reading the definitions for MarketplaceAgreements_Cancel, MarketplaceAgreements_Delete, and single-resource GET, comparing the 2021-01-01 and 2015-06-01 behavior described here. The fix is done when an existing subscription-scoped agreement can be retrieved, cancelled, or deleted after its publisher withdraws the plan, without catalog validation.

Written by the indexing model from the issue text.