(aws-ec2): PrefixList rejects IPv6 entries with an embedded IPv4 address

### Describe the bug

`ec2.PrefixList` with `addressFamily: IP_V6` refuses to synth if any entry embeds an IPv4 address, and accepts a malformed one.

### Regression?

No

### Last known working version

n/a

### Expected Behavior

`::ffff:192.168.0.1/128` and the NAT64 range `64:ff9b::192.0.2.33/96` should be accepted — both are valid IPv6 CIDRs (RFC 4291 §2.5.5, RFC 6052). `::ffff:1dd.1dd.1dd.1dd/128` should be rejected.

### Current Behavior

```
Error: Invalid IPv6 address range: 64:ff9b::192.0.2.33/96
```

and `::ffff:1dd.1dd.1dd.1dd/128` is accepted.

### Reproduction Steps

```ts
new ec2.PrefixList(this, 'PL', {
addressFamily: ec2.AddressFamily.IP_V6,
entries: [{ cidr: '64:ff9b::192.0.2.33/96' }],
});
```

Plain IPv6 entries (`2001:db8::/32`, `64:ff9b::/96`) and IPv4 prefix lists are unaffected.

### Possible Solution

The IPv6 regex in the `PrefixList` constructor (`aws-ec2/lib/prefix-list.ts`) lost its backslashes in the embedded-IPv4 alternatives — `(25[0-5]|2[0-4]d|1dd|[1-9]?d)` matches the literal letter `d` rather than `\d`, and the `.` separator matches any character. Restoring `\d`, `\.` and `\s` fixes it.

### Additional Information/Context

Same root cause as the duplicated copy of this regex in `aws-events/lib/event-pattern.ts`.

### CDK CLI Version

n/a (synth-time validation)

### Framework Version

aws-cdk-lib (current main)

### Node.js Version

20.x

### OS

macOS

### Language

TypeScript

Start in aws-ec2/lib/prefix-list.ts at the PrefixList constructor's IPv6 validation, then compare the duplicated regex in aws-events/lib/event-pattern.ts. Verify that valid embedded-IPv4 IPv6 CIDRs synth successfully while the malformed address is rejected, without changing plain IPv6 or IPv4 prefix-list behavior.

Written by the indexing model from the issue text.