[MULTIPLE_API_VERSION] Detects wrong default tag

From: @qiaozha:
in this case, the readme.md is defined this way, from my understanding the default tag should be package-flexibleserver-2025-08-01, which doesn't have the multiple api version issue, and the tag package-2020-01-01 should be recognized when package-singleservers option exists to satisfy the yaml expression, which shouldn't have existed in the pipeline and this package-2020-01-01 has the version uniform issue.

Now the CI is reporting this multiple api version failure, which indicates it's treating the 2020-01-01 as the default tag, I wonder why?

~~~
``` yaml
title: PostgreSQLManagementClient
description: The Azure Database for PostgreSQL management API provides create, read, update, and delete functionality for Azure PostgreSQL resources including servers, databases, firewall rules, network configuration, security alert policies, log files and configurations with new business model.
openapi-type: arm
tag: package-flexibleserver-2025-08-01
```

``` yaml $(package-singleservers)
tag: package-2020-01-01
```
~~~

### PR
- https://github.com/Azure/azure-rest-api-specs/pull/38799

### Avocado Error

> avocadoForDir: /home/runner/work/azure-rest-api-specs/c93b354fd9c14905bb574a8834c4d69b/specification/postgresql/resource-manager
> {"code":"MULTIPLE_API_VERSION","message":"The default tag contains multiple API versions swaggers.","readMeUrl":"/home/runner/work/azure-rest-api-specs/c93b354fd9c14905bb574a8834c4d69b/specification/postgresql/resource-manager/readme.md","tag":"package-2020-01-01","path":"/home/runner/work/azure-rest-api-specs/c93b354fd9c14905bb574a8834c4d69b/specification/postgresql/resource-manager/readme.md","level":"Error"}

- https://github.com/Azure/azure-rest-api-specs/actions/runs/19358543322/job/55384759524?pr=38799#step:4:45