Fix UnboundLocalError in aws configure set for nested values

[Adityaj0]

Issue

Fixes #10554

Description of changes

aws configure set <section>.<key>.<subkey> <value> crashes with an internal UnboundLocalError and exit code 255 when the nested parent key is not immediately followed by another indented option line:

$ 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

In ConfigFileWriter._update_subattributes, current_indent is only assigned inside the if match is not None: branch, and i is only assigned if the for loop body runs. Both are read unconditionally afterwards:

if (
    starting_indent == current_indent      # unbound if no line matched yet
    or self.SECTION_REGEX.search(line) is not None
):
    ...
else:
    if starting_indent != current_indent:  # unbound if the range was empty
        self._insert_new_values(i, contents, values, '    ')

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

Three valid config layouts trigger it — all three parse correctly via botocore.configloader.raw_config_parse:

[default]        [default]        [default]
s3 =             s3 =             s3 =
# comment
                                  (blank line)

The fix seeds both names before the loop:

current_indent = None
i = index - 1

This is behaviour-preserving for every input that previously worked. current_indent is read only at those two comparison sites, and None never compares equal to starting_indent — which is exactly the "this line does not end the nested block" outcome that a stale-but-different indent already produced for comments appearing later within a block. In the else: clause the comparison was already unconditionally true whenever it was reached, since an equal indent would have broken out of the loop. Seeding i with the nested key's own line makes the values get appended directly after it when nothing follows.

Resulting output for the three cases above:

[default]                                  [default]
s3 =                                       s3 =
# comment                                      signature_version = newval
    signature_version = newval

[default]
s3 =

    signature_version = newval

Testing

Three regression tests added to tests/unit/customizations/configure/test_writer.py, one per layout. Each fails with UnboundLocalError on v2 and passes with the fix.

• tests/unit/customizations/configure/ and tests/functional/configure/: 313 passed
• tests/unit/customizations/: no new failures (the single failure, test_emr_utils.py::TestEMRutils::test_which_with_existing_command, reproduces identically on unmodified v2 in this environment and is unrelated)
• Verified end to end that aws configure set default.s3.max_concurrent_requests 20 now exits 0, and that aws configure get default.s3.max_concurrent_requests reads the value back.

Both touched files already fail ruff format --check/ruff check on unmodified v2, so I deliberately left the surrounding formatting alone rather than growing the diff; the added lines introduce no new findings.