[flake8-pyi] Make example error out-of-the-box (PYI007, PYI008) (#19103)

## Summary

Part of #18972

Both in one PR since they are in the same file

No playground links since the playground does not support rules that
only apply to PYI files

PYI007
---

This PR makes [unrecognized-platform-check
(PYI007)](https://docs.astral.sh/ruff/rules/unrecognized-platform-check/#unrecognized-platform-check-pyi007)'s
example error out-of-the-box

Old example:
```
PS ~\Desktop\New_folder\ruff>echo @"
```
```py
if sys.platform.startswith("linux"):
    # Linux specific definitions
    ...
else:
    # Posix specific definitions
    ...
```
```
"@ | uvx ruff check --isolated --preview --select PYI007 --stdin-filename "test.pyi" -
```
```
All checks passed!
```

New example:
```
PS ~\Desktop\New_folder\ruff>echo @"
```
```py
import sys

if sys.platform is "linux":
    # Linux specific definitions
    ...
else:
    # Posix specific definitions
    ...
```
```
"@ | uvx ruff check --isolated --preview --select PYI007 --stdin-filename "test.pyi" -
```
```snap
test.pyi:3:4: PYI007 Unrecognized `sys.platform` check
  |
1 | import sys
2 |
3 | if sys.platform is "linux":
  |    ^^^^^^^^^^^^^^^^^^^^^^^ PYI007
4 |     # Linux specific definitions
5 |     ...
  |

Found 1 error.
```

Imports were also added to the "use instead" section

> [!NOTE]
> `PYI007` is really hard to trigger, it's only specifically in the case
of a comparison where the operator is not `!=` or `==`. The original
example raises [complex-if-statement-in-stub
(PYI002)](https://docs.astral.sh/ruff/rules/complex-if-statement-in-stub/#complex-if-statement-in-stub-pyi002)
with or without the `import sys`

PYI008
---

This PR makes [unrecognized-platform-name
(PYI008)](https://docs.astral.sh/ruff/rules/unrecognized-platform-name/#unrecognized-platform-name-pyi008)'s
example error out-of-the-box

Old example:
```
PS ~\Desktop\New_folder\ruff>echo @"
```
```py
if sys.platform == "linus": ...
```
```
"@ | uvx ruff check --isolated --preview --select PYI008 --stdin-filename "test.pyi" -
```
```
All checks passed!
```

New example:
```
PS ~\Desktop\New_folder\ruff>echo @"
```
```py
import sys

if sys.platform == "linus": ...
```
```
"@ | uvx ruff check --isolated --preview --select PYI008 --stdin-filename "test.pyi" -
```
```snap
test.pyi:3:20: PYI008 Unrecognized platform `linus`
  |
1 | import sys
2 |
3 | if sys.platform == "linus": ...
  |                    ^^^^^^^ PYI008
  |

Found 1 error.
```

Imports were also added to the "use instead" section

> [!NOTE]
> The original example raises `PYI002` instead

## Test Plan

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

N/A, no functionality/tests affected

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

crates/ruff_linter/src/rules/flake8_pyi/rules/unrecognized_platform.rs

@@ -21,7 +21,9 @@ use crate::registry::Rule;
 ///
 /// ## Example
 /// ```pyi
-/// if sys.platform.startswith("linux"):
+/// import sys
+///
+/// if sys.platform == "xunil"[::-1]:
 ///     # Linux specific definitions
 ///     ...
 /// else:
@@ -31,6 +33,8 @@ use crate::registry::Rule;
 ///
 /// Instead, use a simple string comparison, such as `==` or `!=`:
 /// ```pyi
+/// import sys
+///
 /// if sys.platform == "linux":
 ///     # Linux specific definitions
 ///     ...
@@ -65,11 +69,15 @@ impl Violation for UnrecognizedPlatformCheck {
 ///
 /// ## Example
 /// ```pyi
+/// import sys
+///
 /// if sys.platform == "linus": ...
 /// ```
 ///
 /// Use instead:
 /// ```pyi
+/// import sys
+///
 /// if sys.platform == "linux": ...
 /// ```
 ///