[flake8-async] Make ASYNC220, ASYNC221, and ASYNC222 examples error out-of-the-box (#18978)

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

Part of #18972

All three in one PR since they are in the same file.

This PR makes [create-subprocess-in-async-function
(ASYNC220)](https://docs.astral.sh/ruff/rules/create-subprocess-in-async-function/#create-subprocess-in-async-function-async220)'s
example error out-of-the-box

[Old example](https://play.ruff.rs/465036af-d75f-4bda-ba24-e50e8618bf16)
```py
async def foo():
    os.popen(cmd)
```

[New example](https://play.ruff.rs/8cf43d50-f9e1-45d6-b711-968c7135f2e0)
```py
import os


async def foo():
    os.popen(cmd)
```

Imports were also added to the `Use instead:` section to make it valid
code out-of-the-box.

This PR makes [run-process-in-async-function
(ASYNC221)](https://docs.astral.sh/ruff/rules/run-process-in-async-function/#run-process-in-async-function-async221)'s
example error out-of-the-box

[Old example](https://play.ruff.rs/0698aaa1-c722-4f04-b56c-61edec06945c)
```py
async def foo():
    subprocess.run(cmd)
```

[New example](https://play.ruff.rs/e05bfcbc-e681-4a28-8f50-2c0c2537d038)
```py
import subprocess


async def foo():
    subprocess.run(cmd)
```

Imports were also added to the `Use instead:` section to make it valid
code out-of-the-box.

This PR makes [wait-for-process-in-async-function
(ASYNC222)](https://docs.astral.sh/ruff/rules/wait-for-process-in-async-function/#wait-for-process-in-async-function-async222)'s
example error out-of-the-box

[Old example](https://play.ruff.rs/4305d477-8995-462d-83ae-435731d71e67)
```py
async def foo():
    os.waitpid(0)
```

[New example](https://play.ruff.rs/ad10c042-3b18-49ca-8f5c-5ab720516da1)
```py
import os


async def foo():
    os.waitpid(0)
```

Imports were also added to the `Use instead:` section to make it valid
code out-of-the-box.

## Test Plan

<!-- How was it tested? -->

N/A, no functionality/tests affected

crates/ruff_linter/src/rules/flake8_async/rules/blocking_process_invocation.rs

@@ -21,12 +21,18 @@ use crate::checkers::ast::Checker;
 ///
 /// ## Example
 /// ```python
+/// import os
+///
+///
 /// async def foo():
 ///     os.popen(cmd)
 /// ```
 ///
 /// Use instead:
 /// ```python
+/// import asyncio
+///
+///
 /// async def foo():
 ///     asyncio.create_subprocess_shell(cmd)
 /// ```
@@ -54,12 +60,18 @@ impl Violation for CreateSubprocessInAsyncFunction {
 ///
 /// ## Example
 /// ```python
+/// import subprocess
+///
+///
 /// async def foo():
 ///     subprocess.run(cmd)
 /// ```
 ///
 /// Use instead:
 /// ```python
+/// import asyncio
+///
+///
 /// async def foo():
 ///     asyncio.create_subprocess_shell(cmd)
 /// ```
@@ -87,12 +99,19 @@ impl Violation for RunProcessInAsyncFunction {
 ///
 /// ## Example
 /// ```python
+/// import os
+///
+///
 /// async def foo():
 ///     os.waitpid(0)
 /// ```
 ///
 /// Use instead:
 /// ```python
+/// import asyncio
+/// import os
+///
+///
 /// def wait_for_process():
 ///     os.waitpid(0)
 ///