fix: Windows VSS backup validation to support all selection methods and case-insensitive EC2 detection (#129)

* Initial plan

* Fix Windows VSS validation issues - include all selection sources and case-insensitive regex

Co-authored-by: lgallard <6194359+lgallard@users.noreply.github.com>

* Refactor backup selections handling in main.tf to flatten nested selections for improved resource retrieval

* Enhance backup plan validation by adding lifecycle precondition for Windows VSS backup in main.tf. Update regex for case-insensitive matching of EC2 instances in selection resources. Remove outdated multiple plans example from README.md and clarify usage instructions. Update example error messages for consistency.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: lgallard <6194359+lgallard@users.noreply.github.com>
Co-authored-by: Luis M. Gallardo D <lgallard@gmail.com>

Author: Copilot

.terraform-docs.yml

@@ -36,7 +36,7 @@ content: |-
 
   ## Usage
 
-  You can use this module to create a simple plan using the module's `rule_*` variables. You can also use the `rules` and `selections` list of maps variables to build a more complete plan by defining several rules and selections at once.
+  You can use this module to create a simple plan using the module's `rule_*` variables. You can also use the `rules` and `selections` list of maps variables to build a more complete plan by defining several rules and selections at once. For multiple backup plans, you can use the `plans` variable to create several plans with their own rules and selections.
 
   Check the [examples](/examples/) folder where you can see how to configure backup plans with different selection criteria.
 
@@ -59,6 +59,12 @@ content: |-
   {{ include "examples/complete_plan/main.tf" }}
   ```
 
+  ### Multiple backup plans
+
+  ```hcl
+  {{ include "examples/multiple_plans/main.tf" }}
+  ```
+
 
   ### Simple plan using AWS Organizations backup policies