Set-AzPolicyAssignment Command Loses Description and Display Name

### Description

While testing the Set-AzPolicyAssignment command with the latest version, we noticed the following behavior:

Policy Enforcement Mode and Non-Compliant Messages: These properties are maintained correctly when running the command.

Description and Display Name: These properties are lost when executing the command directly.

### Issue script & Debug output

```PowerShell
# Connect to Azure
Note: Policy assignment is set to DoNotEnforce, for testing.

Get-AzPolicyAssignment -Id "/subscriptions//providers/Microsoft.Authorization/policyAssignments/"

# Set the subscription context
Set-AzContext -SubscriptionId ""

# Assign a policy with the Set-AzPolicyAssignment command
Set-AzPolicyAssignment -Id "/subscriptions//providers/Microsoft.Authorization/policyAssignments/" -EnforcementMode Default
```

### Environment data

```PowerShell
Name Value
---- -----
PSVersion 7.4.2
PSEdition Core
GitCommitId 7.4.2
OS Microsoft Windows 10.0.22635
Platform Win32NT
PSCompatibleVersions {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion 2.3
SerializationVersion 1.1.0.1
WSManStackVersion 3.0
```

### Module versions

```PowerShell
Get-InstalledModule -Name Az -AllVersions

Version Name Repository Description
------- ---- ---------- -----------
12.0.0 Az PSGallery Microsoft Azure PowerShell - Cmdlets to manage resources in Azure. This module is compatib…

Get-InstalledModule -Name Az.Resources -AllVersions

Version Name Repository Description
------- ---- ---------- -----------
7.1.0 Az.Resources PS
```

### Error output

```PowerShell
Example Output Before Changing Enforcement Mode:

Metadata : @{parameterScopes=; createdBy=; createdOn=5/27/2024 7:36:55 PM; updatedBy=; updatedOn=6/24/2024 6:17:53 PM}
NonComplianceMessage :
NotScope :
Parameter : @{profileName=}
Description : This is a test
DisplayName : DiagOpenAI
EnforcementMode : DoNotEnforce
Id : /subscriptions//providers/Microsoft.Authorization/policyAssignments/
IdentityPrincipalId :
IdentityTenantId :
IdentityType :
IdentityUserAssignedIdentity : Microsoft.Azure.PowerShell.Cmdlets.Policy.Models.IdentityUserAssignedIdentities
Location :
Name :
Override : {}
PolicyDefinitionId : /subscriptions//providers/Microsoft.Authorization/policyDefinitions/
ResourceSelector : {}
SystemDataCreatedAt : 5/27/2024 7:36:55 PM
SystemDataCreatedByType : User
SystemDataLastModifiedAt : 6/24/2024 6:17:53 PM
SystemDataLastModifiedByType : User
Type : Microsoft.Authorization/policyAssignments

Example Output After Changing Enforcement Mode:

Metadata : @{createdBy=; createdOn=5/27/2024 7:36:55 PM; updatedBy=; updatedOn=6/24/2024 6:43:50 PM}
NonComplianceMessage :
NotScope :
Parameter : @{profileName=}
Description :
DisplayName :
EnforcementMode : Default
Id : /subscriptions//providers/Microsoft.Authorization/policyAssignments/
IdentityPrincipalId :
IdentityTenantId :
IdentityType :
IdentityUserAssignedIdentity : Microsoft.Azure.PowerShell.Cmdlets.Policy.Models.IdentityUserAssignedIdentities
Location :
Name :
Override :
PolicyDefinitionId : /subscriptions//providers/Microsoft.Authorization/policyDefinitions/
ResourceSelector :
SystemDataCreatedAt : 5/27/2024 7:36:55 PM
SystemDataCreatedByType : User
SystemDataLastModifiedAt : 6/24/2024 6:43:50 PM
SystemDataLastModifiedByType : User
Type : Microsoft.Authorization/policyAssignments

Example of workaround:

Piping the output of Get to the Set:

Get-AzPolicyAssignment -Id "/subscriptions//providers/Microsoft.Authorization/policyAssignments/" | Set-AzPolicyAssignment

Nonetheless, documentation does not state this, so it could be somewhat confusing.
```

Start by reproducing the reported behavior with Get-AzPolicyAssignment and Set-AzPolicyAssignment in Az.Resources 7.1.0, using the enforcement-mode command shown. Compare the direct Set result with the pipeline workaround; done means changing EnforcementMode preserves Description and DisplayName, with regression coverage or documentation explaining the supported behavior.

Written by the indexing model from the issue text.