uuid format incorrectly accepts urn:uuid: prefix

## Bug Description

`format: uuid` should validate plain UUID strings only,
but ajv-formats currently accepts URN-prefixed UUIDs like `urn:uuid:550e8400-e29b-41d4-a716-446655440000` as valid.

## Root Cause

The regex at `src/formats.ts:73` contains an optional non-capturing group `(?:urn:uuid:)?` that allows the URN prefix:
```ts
uuid: /^(?:urn:uuid:)?[0-9a-f]{8}-(?:[0-9a-f]{4}-){3}[0-9a-f]{12}$/i,
```

## Why This Is Wrong

JSON Schema Validation spec §7.3.5 explicitly states:

> "Note also that the 'uuid' format is for plain UUIDs,
> not UUIDs in URNs. For UUIDs as URNs, use the 'uri' format."

The `urn:uuid:` prefix belongs to `format: uri` validation, not `format: uuid`.

## Steps to Reproduce
```js
const Ajv = require("ajv")
const addFormats = require("ajv-formats")

const ajv = new Ajv()
addFormats(ajv)

const validate = ajv.compile({
format: "uuid"
})

// This should be false but returns true
console.log(validate("urn:uuid:550e8400-e29b-41d4-a716-446655440000"))
```

## Expected Behavior
`false` - URN-prefixed UUIDs are not plain UUIDs

## Actual Behavior
`true` - URN prefix is silently accepted

## Fix

Remove the `(?:urn:uuid:)?` group from the regex:
```ts
// Before
uuid: /^(?:urn:uuid:)?[0-9a-f]{8}-(?:[0-9a-f]{4}-){3}[0-9a-f]{12}$/i,

// After
uuid: /^[0-9a-f]{8}-(?:[0-9a-f]{4}-){3}[0-9a-f]{12}$/i,
```

## References
- JSON Schema Validation spec §7.3.5
- RFC 4122 §3 (plain UUID text representation)
- RFC 9562 §4 (same text representation, obsoletes RFC 4122)