feat: implement expiration policy logic for prebuilds (#17996)

## Summary 

This PR introduces support for expiration policies in prebuilds. The TTL
(time-to-live) is retrieved from the Terraform configuration
([terraform-provider-coder
PR](https://github.com/coder/terraform-provider-coder/pull/404)):
```
prebuilds = {
	  instances = 2
	  expiration_policy {
		  ttl = 86400
	  }
  }
```
**Note**: Since there is no need for precise TTL enforcement down to the
second, in this implementation expired prebuilds are handled in a single
reconciliation cycle: they are deleted, and new instances are created
only if needed to match the desired count.

## Changes

* The outcome of a reconciliation cycle is now expressed as a slice of
reconciliation actions, instead of a single aggregated action.
* Adjusted reconciliation logic to delete expired prebuilds and
guarantee that the number of desired instances is correct.
* Updated relevant data structures and methods to support expiration
policies parameters.
* Added documentation to `Prebuilt workspaces` page
* Update `terraform-provider-coder` to version 2.5.0:
https://github.com/coder/terraform-provider-coder/releases/tag/v2.5.0

Depends on: https://github.com/coder/terraform-provider-coder/pull/404
Fixes: https://github.com/coder/coder/issues/17916

docs/admin/templates/extending-templates/prebuilt-workspaces.md

@@ -31,7 +31,8 @@ Prebuilt workspaces are tightly integrated with [workspace presets](./parameters
 ## Enable prebuilt workspaces for template presets
 
 In your template, add a `prebuilds` block within a `coder_workspace_preset` definition to identify the number of prebuilt
-instances your Coder deployment should maintain:
+instances your Coder deployment should maintain, and optionally configure a `expiration_policy` block to set a TTL
+(Time To Live) for unclaimed prebuilt workspaces to ensure stale resources are automatically cleaned up.
 
    ```hcl
    data "coder_workspace_preset" "goland" {
@@ -42,7 +43,10 @@ instances your Coder deployment should maintain:
        memory        = 16
      }
      prebuilds {
-       instances = 3  # Number of prebuilt workspaces to maintain
+       instances = 3   # Number of prebuilt workspaces to maintain
+       expiration_policy {
+          ttl = 86400  # Time (in seconds) after which unclaimed prebuilds are expired (1 day)
+      }
      }
    }
    ```
@@ -50,6 +54,9 @@ instances your Coder deployment should maintain:
 After you publish a new template version, Coder will automatically provision and maintain prebuilt workspaces through an
 internal reconciliation loop (similar to Kubernetes) to ensure the defined `instances` count are running.
 
+The `expiration_policy` block ensures that any prebuilt workspaces left unclaimed for more than `ttl` seconds is considered
+expired and automatically cleaned up.
+
 ## Prebuilt workspace lifecycle
 
 Prebuilt workspaces follow a specific lifecycle from creation through eligibility to claiming.
@@ -95,6 +102,15 @@ Unclaimed prebuilt workspaces can be interacted with in the same way as any othe
 However, if a Prebuilt workspace is stopped, the reconciliation loop will not destroy it.
 This gives template admins the ability to park problematic prebuilt workspaces in a stopped state for further investigation.
 
+### Expiration Policy
+
+Prebuilt workspaces support expiration policies through the `ttl` setting inside the `expiration_policy` block.
+This value defines the Time To Live (TTL) of a prebuilt workspace, i.e., the duration in seconds that an unclaimed
+prebuilt workspace can remain before it is considered expired and eligible for cleanup.
+
+Expired prebuilt workspaces are removed during the reconciliation loop to avoid stale environments and resource waste.
+New prebuilt workspaces are only created to maintain the desired count if needed.
+
 ### Template updates and the prebuilt workspace lifecycle
 
 Prebuilt workspaces are not updated after they are provisioned.