fix(cleaners): prevent UnboundLocalError in _get_indexed_match when pattern is absent

[Tyagiquamar]

Closes #4427

Problem

When extract_text_before() or extract_text_after() was called with a pattern that had no matches in the input string, the loop in _get_indexed_match() never ran. As a result, the loop variable i was never bound, causing UnboundLocalError when building the ValueError message instead of raising the intended descriptive ValueError.

Solution

1. Initialized largest_index = -1 before the iteration loop.
2. In the error message construction when regex_match is None, check if largest_index < 0 and raise a clear ValueError("Result with index {index} was not found. No matches were found.").
3. Added unit tests for zero-match cases across _get_indexed_match, extract_text_before, and extract_text_after in test_unstructured/cleaners/test_extract.py.