[Key Vault] Update continuation tokens

[mccoyp]

Description

Updates Admin continuation tokens to match the new format from Core.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[Copilot]

Pull request overview

This pull request updates the continuation token format in the Azure Key Vault Administration library to match the new format from Azure Core. The change replaces pickle-based serialization with JSON-based serialization for improved security and compatibility.

Changes:

• Updated minimum azure-core dependency from 1.31.0 to 1.38.0
• Replaced pickle serialization with JSON serialization for continuation tokens
• Added new _get_continuation_token() helper function that creates JSON-based tokens with versioned schema
• Modernized super() call in KeyVaultBackupClientPolling
• Added comprehensive documentation for the continuation token approach

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
sdk/keyvault/azure-keyvault-administration/setup.py	Bumped minimum azure-core version to 1.38.0
sdk/keyvault/azure-keyvault-administration/azure/keyvault/administration/_backup_client.py	Replaced pickle with json import; added _get_continuation_token() function implementing new JSON-based token format; added type annotations to _parse_status_url()
sdk/keyvault/azure-keyvault-administration/azure/keyvault/administration/aio/_backup_client.py	Removed pickle import; imported and used _get_continuation_token() from sync module
sdk/keyvault/azure-keyvault-administration/azure/keyvault/administration/_internal/polling.py	Modernized super() call; added comprehensive docstring to get_continuation_token() method
sdk/keyvault/azure-keyvault-administration/CHANGELOG.md	Documented continuation token format change and azure-core version update