Add Key Vault ID as a tag to virtual machines and enhance variable descriptions

[lixhunter]

Description

This pull request clarifies and enforces the relationship between the key_vault_id and store_secret_in_key_vault variables, ensures correct tagging behavior for virtual machines, and adds targeted tests for these behaviors. The changes improve both the documentation and the implementation to prevent misconfiguration and make the module's behavior more predictable.

Variable validation and documentation improvements:

• Added explicit validation to variables.tf to ensure key_vault_id must be set when store_secret_in_key_vault is true and must be null when store_secret_in_key_vault is false. Updated variable descriptions for both key_vault_id and store_secret_in_key_vault for clarity. [1] [2]
• Updated the README.md to clarify the relationship and requirements between key_vault_id and store_secret_in_key_vault. [1] [2]

Tagging logic enhancements:

• Refactored tag logic in r-vm.tf to add the key_vault_id as a tag to the virtual machine only when it is set, using a new local.tags_virtual_machine local value. Updated both Linux and Windows VM resources to use this logic. [1] [2] [3]

Testing improvements:

• Added a new test file input_key_vault_id.tftest.hcl to verify that the key_vault_id tag is only present when expected, and never present when it should not be.
• Updated existing tests to explicitly set key_vault_id = null when store_secret_in_key_vault = false, ensuring test coverage for the new validation logic. [1] [2] [3]

PR Checklist

• I have checked if my changes close any open issues. If so please include appropriate closing keywords below.
• I have updated/added documentation written in a helpful and kind way to assist users that may be unfamiliar with the resource / data source.
• I have checked for a proper tag for this PR: breaking-change, feature, fix, other, ignore-release
• I have used a meaningful PR title to help maintainers and other users understand this change and help prevent duplicate work.

Motivation and Context

Breaking Changes

[rswrz]

Nice work, thanks for implementing this enhancement.