aws configure set: UnboundLocalError when nested key is followed by a comment, blank line, or end of file

### Describe the bug

`aws configure set` crashes with an internal `UnboundLocalError` when writing a nested (sub-attribute) value into a config file where the parent key is not immediately followed by another indented option line.

The error surfaces to the user as a bare Python message with no indication of what is wrong with their config:

```
aws: [ERROR]: cannot access local variable 'current_indent' where it is not associated with a value
```

The command exits `255` and the config file is left unmodified.

### Regression Issue

- [ ] Select this option if this issue appears to be a regression.

### Expected Behavior

`aws configure set default.s3.max_concurrent_requests 20` should add the nested value under the existing `s3 =` key and exit `0`.

### Current Behavior

The command aborts with `UnboundLocalError` and exit code `255`.

Traceback (with `--debug` stripped to the relevant frames):

```
File "awscli/customizations/configure/writer.py", line 113, in update_config
self._update_section_contents(contents, section_name, new_values)
File "awscli/customizations/configure/writer.py", line 210, in _update_section_contents
j = self._update_subattributes(
File "awscli/customizations/configure/writer.py", line 246, in _update_subattributes
starting_indent == current_indent
UnboundLocalError: cannot access local variable 'current_indent' where it is not associated with a value
```

### Reproduction Steps

Any of the following three config files reproduce it. All are valid and all parse correctly with `botocore`.

**1. Parent key followed by a comment**

```ini
[default]
region = us-east-1
s3 =
# tune these later
```

**2. Parent key is the last line in the file**

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

**3. Parent key followed by a blank line**

```ini
[default]
s3 =

```

Then run:

```console
$ AWS_CONFIG_FILE=./config aws configure set default.s3.max_concurrent_requests 20

aws: [ERROR]: cannot access local variable 'current_indent' where it is not associated with a value
$ echo $?
255
```

### Possible Solution

In `ConfigFileWriter._update_subattributes`, `current_indent` is only assigned inside the `if match is not None:` branch, but it is read unconditionally a few lines later:

```python
def _update_subattributes(self, index, contents, values, starting_indent):
index += 1
for i in range(index, len(contents)):
line = contents[i]
match = self.OPTION_REGEX.search(line)
if match is not None:
current_indent = len(match.group(1)) - len(match.group(1).lstrip())
...
if (
starting_indent == current_indent # <-- may be unbound
or self.SECTION_REGEX.search(line) is not None
):
...
else:
if starting_indent != current_indent: # <-- may be unbound
self._insert_new_values(i, contents, values, ' ')
return i
```

Comment and blank lines do not match `OPTION_REGEX`, so if the first line after the parent key is one of those, `current_indent` is never bound. If the parent key is the last line of the file the `range()` is empty, so both `current_indent` and `i` are unbound when the `else:` clause runs.

Seeding `current_indent = None` and `i = index - 1` before the loop fixes all three cases and preserves existing behaviour: `None` never compares equal to `starting_indent`, which is exactly the "this line does not end the nested block" behaviour that a stale indent value produced for comments appearing later in a block.

I have a PR ready with a fix and regression tests for all three cases.

### CLI version used

aws-cli/2.36.23 (reproduced on `v2` at 4c331fc)

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

macOS 15 (Darwin 25.2.0), Python 3.12