[AWS::MemoryDB::ParameterGroup] Parameter Group not applied on Create, but on Update

### Name of the resource

AWS::MemoryDB::ParameterGroup

### Resource Name

_No response_

### Issue Description

When an `AWS::MemoryDB::ParameterGroup` resource is created with a `Parameters`
map, CloudFormation creates the parameter group but **does not apply the
parameter values**. The stack reports `CREATE_COMPLETE`, and no error, warning
or event indicates that the values were dropped.

The same template, applied as an **update** with a changed value, applies it
correctly.

The result is a parameter group that looks correct in the template and in the
CloudFormation console, but is empty in the MemoryDB API.

## Environment

| | |
|---|---|
| Region | `eu-west-2` |
| Resource | `AWS::MemoryDB::ParameterGroup` |
| Family | `memorydb_valkey7` |
| Parameter | `notify-keyspace-events` |
| Deployed via | `aws cloudformation deploy` (AWS CLI v2) |

### Expected Behavior

```
[
{
"Name": "notify-keyspace-events",
"Value": "KEg$",
"Description": "The keyspace events for Redis to notify Pub/Sub clients about. By default all notifications are disabled",
"DataType": "string",
"MinimumEngineVersion": "7.2.6"
}
]
```
directly after Create

### Observed Behavior

```
[
{
"Name": "notify-keyspace-events",
"Description": "The keyspace events for Redis to notify Pub/Sub clients about. By default all notifications are disabled",
"DataType": "string",
"MinimumEngineVersion": "7.2.6"
}
]
```
directly after Create

```
[
{
"Name": "notify-keyspace-events",
"Value": "KEg$",
"Description": "The keyspace events for Redis to notify Pub/Sub clients about. By default all notifications are disabled",
"DataType": "string",
"MinimumEngineVersion": "7.2.6"
}
]
```
after Update

### Test Cases

`pgtest2.yaml` — the entire template. No cluster, no subnet group, no other
resource:

```yaml
AWSTemplateFormatVersion: '2010-09-09'
Resources:
Pg:
Type: AWS::MemoryDB::ParameterGroup
Properties:
ParameterGroupName: cfn-pgtest2-keyspace
Family: memorydb_valkey7
Description: repro
Parameters:
notify-keyspace-events: 'KEg$'
```

### Step 1 — create the stack

```bash
aws cloudformation deploy --stack-name cfn-pgtest2 --template-file pgtest2.yaml
# Successfully created/updated stack - cfn-pgtest2

aws memorydb describe-parameters \
--parameter-group-name cfn-pgtest2-keyspace \
--query 'Parameters[?Name==`notify-keyspace-events`]' \
--output json
```

**Actual output:**

```json
[
{
"Name": "notify-keyspace-events",
"Description": "The keyspace events for Redis to notify Pub/Sub clients about. By default all notifications are disabled",
"DataType": "string",
"MinimumEngineVersion": "7.2.6"
}
]
```

There is **no `Value` field at all** — not `""`, not a staged or pending value.
The parameter is simply unset.

**Expected output:** the same object with `"Value": "KEg$"`.

### Step 2 — control: apply the value directly

Against the **same** parameter group created by CloudFormation above:

```bash
aws memorydb update-parameter-group \
--parameter-group-name cfn-pgtest2-keyspace \
--parameter-name-values 'ParameterName=notify-keyspace-events,ParameterValue=KEg$'

aws memorydb describe-parameters \
--parameter-group-name cfn-pgtest2-keyspace \
--query 'Parameters[?Name==`notify-keyspace-events`]' \
--output json
```

### Other Details

_No response_

Use pgtest2.yaml as the minimal reproduction; run the listed aws cloudformation deploy and aws memorydb describe-parameters commands, then compare with aws memorydb update-parameter-group. Done when creation applies notify-keyspace-events and describe-parameters returns its Value.

Written by the indexing model from the issue text.