httpx encodes header values as ASCII, so a smart quote, non-breaking
space or byte-order mark pasted into LLM_API_KEY (or LLM_EXTRA_HEADERS)
surfaced as a bare UnicodeEncodeError from inside the client, wrapped in
ModelConnectionError. The preflight now checks these settings first and
fails with the setting name, the code point, its Unicode name and its
position. Nothing is trimmed or rewritten and the key itself is never
printed.
