Add Validation for Auto-Generated SDK PR before PR review & merge

The goal is to ensure that any auto-generated SDK PRs for SDK release is ready for review before it enters the review phrase. This validation step is critical for maintaining release quality and avoiding unnecessary iterations.

Checks to Include

Spec Merged in Public Main Repo
- Confirm that the release starts with a specification already merged into the public main repository.
the violation: https://github.com/Azure/azure-sdk-for-go/pull/25441 spec PR not merged.

Target Latest Version
- Ensure the release targets the latest preview or GA version as applicable.
The violation: https://github.com/Azure/sdk-release-request/issues/6672 use a early api-version

Use the approved namespace.
- Need to ensure service teams are using the approved namespace for SDK generation and release.
- Face a case that service team need to go back and update namespace after PR is generated and approved. https://github.com/Azure/azure-rest-api-specs/pull/41734

Whether the check happens in the Release Planner or in the pipeline is an implementation detail and should not affect the goal of this issue.
so I will let @weshaggard and @maririos to decide the detail.

Start by locating the Release Planner and pipeline entry points in azure-sdk-tools, then compare the linked violating PRs and issue with the three checks listed here. Done means auto-generated SDK release PRs are blocked or flagged before review when the spec is unmerged, the API version is not latest, or the namespace is unapproved.

Written by the indexing model from the issue text.