CcToolchainConfigInfo variable libraries_to_link has no field shared_libraries

## Description of the bug:
I learn to write cc-toolchain from [cc-toolchain-config-reference](https://bazel.build/versions/8.3.0/docs/cc-toolchain-config-reference#flag-groups), I want to add some flags before each shared library like `-Wl,--as-needed foo.so`, so I add a feature
```
feature(
name = "default_linker_flags",
enabled = True,
flag_sets = [
flag_set(
actions = [
ACTION_NAMES.cpp_link_executable,
ACTION_NAMES.cpp_link_dynamic_library,
ACTION_NAMES.cpp_link_nodeps_dynamic_library,
],
flag_groups = ([
flag_group(
flags = ["-lstdc++"],
),
flag_group (
iterate_over = "libraries_to_link",
flag_groups = [
flag_group (
iterate_over = "libraries_to_link.shared_libraries",
flags = ["-Wl,--as-needed", "%{libraries_to_link.shared_libraries.path}"],
),
],
)
]),
),
],
),
```
but get
```
Linking hello_lib.so failed: Invalid toolchain configuration: Cannot expand variable 'libraries_to_link.shared_libraries': structure libraries_to_link doesn't have a field named 'shared_libraries'
```

finally I solve it by
```
flag_group (
iterate_over = "libraries_to_link",
flag_groups = [
flag_group(
flags = ["-Wl,--as-needed", "%{libraries_to_link.path}"],
expand_if_equal = variable_with_value(
name = "libraries_to_link.type",
value = "versioned_dynamic_library",
),
),
],
expand_if_available = "libraries_to_link",
),
```
and
```
flag_group (
iterate_over = "libraries_to_link",
flags = ["-Wl,--as-needed", "%{libraries_to_link.path}"],
expand_if_equal = variable_with_value(
name = "libraries_to_link.type",
value = "versioned_dynamic_library",
),
),
```
doesn't work

### Which category does this issue belong to?

C++ Rules

### What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

Repo with an empty MODULE.bazel and:

BUILD.bazel:

```
cc_binary(
name = "hello_world",
srcs = ["hello_world.cpp"],
deps = [":hello_lib_import"],
)

cc_binary(
name = "hello_lib.so",
srcs = ["hello_lib.cpp"],
linkshared = True,
copts = ["-fPIC"],
visibility = ["//visibility:public"],
)

cc_import(
name = "hello_lib_import",
shared_library = ":hello_lib.so",
hdrs = [],
visibility = ["//visibility:public"],
)
```
hello_lib.cpp
```
#include

extern "C" void hello_from_lib() {
std::cout << "Hello from shared library!" << std::endl;
}
```

hello_world.cpp

```
#include

extern "C" void hello_from_lib();

int main() {
std::cout << "Hello, Bazel!" << std::endl;
hello_from_lib();
return 0;
}
```

### Which operating system are you running Bazel on?

ubuntu 18.04

### What is the output of `bazel info release`?

8.3.0

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse HEAD` ?

```text

```

### If this is a regression, please try to identify the Bazel commit where the bug was introduced with bazelisk --bisect.

_No response_

### Have you found anything relevant by searching the web?

_No response_

### Any other information, logs, or outputs that you want to share?

_No response_

Start with the cc-toolchain-config-reference and reproduce the failure using the provided BUILD.bazel and C++ sources on Bazel 8.3.0. Trace how the feature's flag_group expands libraries_to_link and its available fields, then verify the behavior against the supplied shared-library example. Done means the documented variable structure and actual expansion behavior agree, with the reported configuration no longer failing unexpectedly.

Written by the indexing model from the issue text.