Automate consolidated notification for autogenerated SDK PRs

## Summary
Implement a consolidated automated email notification for autogenerated SDK pull requests.

## Goal
Send one notification that summarizes all SDK PRs generated for a release plan, instead of sending separate emails per PR.

## Audience
- Release plan owner
- SDK code owner
- Shanghai SDK team
- azsdk engagement exp support

## Requirements
- Collect all autogenerated SDK PRs associated with the same release plan.
- Send exactly one consolidated email when the last SDK PR is linked to the release plan.
- Include the full list of generated PRs in the email, with enough context for recipients to review status and next actions.
- Ensure the notification is not sent prematurely before all expected autogenerated SDK PRs are linked.
- Ensure duplicate consolidated emails are not sent for the same set of autogenerated SDK PRs unless a defined retry scenario requires it.

## Suggested Acceptance Criteria
- One email is sent per release plan after the last autogenerated SDK PR is linked.
- The email contains all related SDK PR references.
- Recipient resolution includes the intended audience.
- Duplicate notification prevention is implemented.
- Logging and retry behavior are defined for notification failures.

## Notes
This sub-issue covers the consolidated notification behavior for autogenerated SDK PR creation.