Gazelle not raising error when a target py_test exists in place of a py_binary

# 🐞 bug report

### Affected Rule

rules_python_gazelle_plugin functionality

### Is this a regression?

Still needs assessment

### Description
When gazelle runs over a BUILD file and finds a py_test target where it wants to make a py_binary target, the gazelle extension flags the conflict, but it does not elevate to an error. This is not the case for other target conflicts. For example, if a py_test exists where a py_library should, a full error is raised and the exit code respects this.

When this conflict happens, no other gazelle edits are recommended for the affected target. Recommendations to other targets proceed as normal.

## 🔬 Minimal Reproduction

Make a simple tree like
```
lib/demo/
- example_lib.py
- example_bin.py
- example_test.py
```
The contents of each are
example_bin.py
```
from lib.demo import example_lib

def main() -> None:
print("Lib call:", example_lib.some_helper())

if __name__ == "__main__":
main()
```
example_lib.py
```
def some_helper() -> int:
return 52
```
example_test.py
```
import unittest

from lib.demo import example_lib

def test_thing() -> None:
assert example_lib.some_helper() == 52

if __name__ == "__main__":
unittest.main()
```
Now make the `BUILD` file with the following contents
```
load("@rules_python//python:defs.bzl", "py_library", "py_test")

py_test(
name = "example_bin",
srcs = ["example_bin.py"],
)

py_library(
name = "example_lib",
srcs = ["example_lib.py"],
)

py_test(
name = "example_test",
srcs = ["example_test.py"],
)
```
Run `bazel run //:gazelle -- -strict -mode diff` and see the following output
```
$ bazel run //:gazelle -- -strict -mode fix
(21:34:13) INFO: Current date is 2026-02-16
(21:34:13) INFO: Analyzed target //:gazelle (0 packages loaded, 0 targets configured).
(21:34:13) INFO: Found 1 target...
Target //:gazelle up-to-date:
bazel-bin/gazelle-runner.bash
bazel-bin/gazelle
(21:34:13) INFO: Elapsed time: 0.129s, Critical Path: 0.05s
(21:34:13) INFO: 1 process: 4 action cache hit, 1 internal.
(21:34:13) INFO: Build completed successfully, 1 total action
gazelle: failed to generate target "//lib/demo:example_bin" of kind "py_binary": a target of kind "py_test" with the same name already exists
```
Now notice the difference in error when `py_library` is converted incorrectly to a `py_test`.
```
gazelle: failed to generate target "//lib/demo:example_bin" of kind "py_binary": a target of kind "py_test" with the same name already exists
gazelle: ERROR: failed to generate target "//lib/demo:example_lib" of kind "py_library": a target of kind "py_test" with the same name already exists. Use the '# gazelle:python_library_naming_convention' directive to change the naming convention.
```
The details are the same, but the latter scenario is reflected in the exit code with a value of 1.

## 🌍 Your Environment

**Operating System:**
debian linux

**Output of `bazel version`:**
8.4.2

**Rules_python version:**
1.8.3

Start at the rules_python_gazelle_plugin entry point that handles conflicting generated targets, using the supplied lib/demo reproduction and `bazel run //:gazelle -- -strict -mode fix` command. Compare the py_binary-versus-py_test conflict with the py_library-versus-py_test case. Done means the py_binary conflict is reported as an error and the command exits with status 1, consistently with the other conflict.

由索引模型根据 Issue 内容生成。