az aks namespace update --annotations replaces existing annotations instead of updating incrementally

### Describe the bug

The az aks namespace update --annotations command replaces all existing annotations on a managed namespace with only the values provided via the --annotations parameter.
This behavior prevents users from adding or updating individual annotations without first retrieving and re-specifying the entire existing annotation set.

### Related command

az aks namespace update --annotations

### Errors

N/A

### Issue script & Debug output

#### Variables
RG=my-resource-group
CLUSTER=my-cluster-name
NS=my-namespace

#### Step 1: Set initial annotations
az aks namespace update -g $RG --cluster-name $CLUSTER -n $NS --annotations a=1 b=2 --debug

#### Step 2: Verify annotations
az aks namespace show -g $RG --cluster-name $CLUSTER -n $NS --debug

#### Step 3: Add a new annotation (expected: merge, actual: replace)
az aks namespace update -g $RG --cluster-name $CLUSTER -n $NS --annotations c=3 --debug

#### Step 4: Verify annotations again
az aks namespace show -g $RG --cluster-name $CLUSTER -n $NS --debug

#### Annotations after Step 4:
"annotations": {
"c": "3"
},

### Expected behavior

#### Expected Annotations after Step 4:
"annotations": {
"a": "1",
"b": "2",
"c": "3"
},

### Environment Summary

azure-cli 2.87.0

core 2.87.0
telemetry 1.1.0

Extensions:
resource-graph 2.1.1

Dependencies:
msal 1.36.0
azure-mgmt-resource 24.0.0

Python location '/opt/homebrew/Cellar/azure-cli/2.87.0/libexec/bin/python'
Config directory '/Users/sunehabose/.azure'
Extensions directory '/Users/sunehabose/.azure/cliextensions'

Python (Darwin) 3.13.13 (main, Apr 7 2026, 18:19:01) [Clang 21.0.0 (clang-2100.0.123.102)]

### Additional context

_No response_