aws configure set s3.* can raise UnboundLocalError when updating nested config blocks

### Describe the bug

`aws configure set s3.signature_version s3v4` can crash when the target config already has a nested `s3 =` block and the next line is not another option line.

I reproduced this with all of the following existing config shapes:

- an empty nested block
- a comment immediately after `s3 =`
- an immediate next section header after `s3 =`

The crash comes from `ConfigFileWriter._update_subattributes()` reading `current_indent` even when `OPTION_REGEX` did not match the current line.

### Regression Issue

This does not look like a recent regression. `git blame` shows `_update_subattributes()` was introduced with this behavior in `9c2fd8098a`.

### Expected Behavior

Updating a nested config value should append or update the nested key without crashing.

For example, starting from:

```ini
[default]
s3 =
# comment
addressing_style = path
```

setting `s3.signature_version` should produce:

```ini
[default]
s3 =
# comment
addressing_style = path
signature_version = s3v4
```

### Current Behavior

An uncaught traceback is raised:

```text
Traceback (most recent call last):
File "/Users/hoangvu/Code/OSS/aws-cli/awscli/customizations/configure/writer.py", line 115, in update_config
self._update_section_contents(contents, section_name, new_values)
File "/Users/hoangvu/Code/OSS/aws-cli/awscli/customizations/configure/writer.py", line 203, in _update_section_contents
j = self._update_subattributes(
File "/Users/hoangvu/Code/OSS/aws-cli/awscli/customizations/configure/writer.py", line 231, in _update_subattributes
if starting_indent == current_indent or \
^^^^^^^^^^^^^^
UnboundLocalError: cannot access local variable 'current_indent' where it is not associated with a value
```

### Reproduction Steps

Minimal reproducer:

```python
from awscli.customizations.configure.writer import ConfigFileWriter
import os
import tempfile

fd, path = tempfile.mkstemp()
os.close(fd)

with open(path, "w") as f:
f.write("[default]\ns3 =\n# comment\n addressing_style = path\n")

ConfigFileWriter().update_config(
{"__section__": "default", "s3": {"signature_version": "s3v4"}},
path,
)
```

This also reproduces with:

```ini
[default]
s3 =
```

and:

```ini
[default]
s3 =
[profile other]
region = us-east-1
```

### Possible Solution

Handle non-option lines before comparing indentation, or initialize `current_indent` safely.

For example, `_update_subattributes()` could skip lines where `OPTION_REGEX` does not match, while still treating a new section header as the boundary for inserting new nested values.

It would also be helpful to add regression tests in `tests/unit/customizations/configure/test_writer.py` for:

- empty nested blocks
- comments after `s3 =`
- section headers immediately after `s3 =`

### Additional Information/Context

I reproduced this against the current `develop` checkout:

- `aws-cli/1.45.14`
- `develop` @ `b698a6a77`
- macOS 26.5 arm64
- Python 3.13.2

The user-facing path appears to be `aws configure set s3.signature_version ...`, but the minimal reproducer above avoids unrelated environment/setup dependencies.

### CLI version used

aws-cli/1.45.14

### Environment details (OS name and version, etc.)

macOS 26.5 arm64, Python 3.13.2