Fix the config reader for a settings file

You asked an AI for a function that reads one setting from a config file. Its first version worked on a clean file and handled nothing else: no comments, no blank lines, no broken lines. The new version reads key=value lines defensively and falls back to a default.

An assistant wrote read_config_value(lines, key, fallback). It is in the editor. It has a bug. Find it and fix it.

Input and output:

• lines: the lines of the config file, in order.
• key: the setting to read.
• fallback: the value to return when the setting is not found.
• Return the setting's value as a string.

Rules the tests check:

• To trim text means to remove spaces, tabs, newlines and carriage returns from both ends, and nothing else.
• Trim key before comparing.
• For each line in order: trim it. Skip it if it is empty or starts with #.
• Split the line at its first =. Skip the line if it has no =. The key is the part before the first =, trimmed. The value is everything after the first =, trimmed.
• Skip a line whose key is empty.
• A line matches when its key equals key exactly, case included. When several lines match, the last one wins.
• Return the value of the last match, even if it is empty. With no match, return fallback unchanged.