Gazelle: `python_default_visibility` Not Applied to `py_test` Targets

## Description

The `# gazelle:python_default_visibility` directive is not being applied to `py_test` targets, even though it correctly applies to `py_library` and `py_binary` targets. This occurs when using `# gazelle:map_kind` to map to custom rule macros.

## Environment

- **rules_python version**: Latest (tested with rules_python_gazelle_plugin)
- **Gazelle version**: Latest
- **Bazel version**: 8.0+
- **Operating System**: Linux

## Reproduction Steps

1. Create a BUILD file with the following directives:

```starlark
# gazelle:map_kind py_binary py_binary @aspect_rules_py//py:defs.bzl
# gazelle:map_kind py_library py_library @aspect_rules_py//py:defs.bzl
# gazelle:map_kind py_test py_test @aspect_rules_py//py:defs.bzl
# gazelle:python_default_visibility //visibility:public
```

2. Run `bazel run //:gazelle`

3. Observe generated BUILD files

## Expected Behavior

Both `py_library` and `py_test` targets should have `visibility = ["//visibility:public"]` added:

```starlark
py_library(
name = "mylib",
srcs = ["mylib.py"],
visibility = ["//visibility:public"], # ✓ Generated correctly
)

py_test(
name = "mylib_test",
srcs = ["mylib_test.py"],
visibility = ["//visibility:public"], # ✗ NOT generated
deps = [":mylib"],
)
```

## Actual Behavior

Only `py_library` targets get the visibility attribute. `py_test` targets are generated without any visibility:

```starlark
py_library(
name = "mylib",
srcs = ["mylib.py"],
visibility = ["//visibility:public"], # ✓ Generated correctly
)

ctc_py_test(
name = "mylib_test",
srcs = ["mylib_test.py"],
# visibility is missing
deps = [":mylib"],
)
```

## Impact

This causes visibility errors when test targets need to be referenced as dependencies elsewhere in the build graph (e.g., for creating development virtual environments that include test dependencies).

Example error:
```
ERROR: /path/to/BUILD:12:12: in _py_venv_binary rule //ctc_pylibs:dev_venv:
Visibility error: target '//ctc_pylibs/auth:vault_test' is not visible from
target '//ctc_pylibs:dev_venv'
```

## Root Cause

In `rules_python_gazelle_plugin/python/generate.go`, the visibility is correctly retrieved from configuration:

```go
visibility := cfg.Visibility()
```

However, when generating `py_test` targets, the `newPyTestTargetBuilder` function (line ~412) creates targets without calling `addVisibility(visibility)`:

```go
newPyTestTargetBuilder := func(srcs *treeset.Set, pyTestTargetName string) *targetBuilder {
// ... parsing logic ...
return newTargetBuilder(pyTestKind, pyTestTargetName, pythonProjectRoot, args.Rel, pyFileNames, cfg.ResolveSiblingImports()).
addSrcs(srcs).
addModuleDependencies(deps).
addResolvedDependencies(annotations.includeDeps).
setAnnotations(*annotations).
generateImportsAttribute()
// ❌ Missing: .addVisibility(visibility)
}
```

In contrast, `py_library` and `py_binary` targets correctly call `addVisibility(visibility)`.

## Proposed Fix

Add `.addVisibility(visibility)` to the test target builder in `generate.go`:

```go
return newTargetBuilder(pyTestKind, pyTestTargetName, pythonProjectRoot, args.Rel, pyFileNames, cfg.ResolveSiblingImports()).
addSrcs(srcs).
addModuleDependencies(deps).
addResolvedDependencies(annotations.includeDeps).
setAnnotations(*annotations).
generateImportsAttribute().
addVisibility(visibility) // ← Add this line
```

This should be added in both the per-package test generation path (line ~438) and the per-file test generation path (line ~460).

## Additional Context

The `python_default_visibility` directive is documented to apply to "all python targets" according to the [directives documentation](https://rules-python.readthedocs.io/en/latest/gazelle/docs/directives.html#python-default-visibility), but in practice it only applies to non-test targets.

Similarly, the `python_visibility` directive (which appends additional visibility labels) also does not affect test targets.