Error classification substring-matches reqwest Debug output; fragile across dependency bumps #480

🤖 Found by Claude ultrareview — automated high-effort code review. Please verify independently before acting.

Location: impit/src/errors.rs:115 and :125 (ImpitError::from)

Connect-timeout and incomplete-body classification relies on substring-matching the reqwest error's Debug output:

... format!("{:?}", error).to_lowercase().contains("connection timed out") ...
... format!("{:?}", error).to_lowercase().contains("unexpectedeof") ...

Impact: the Debug formatting of hyper/reqwest internal error types is not a stable API. A dependency bump can change the string, after which every connect-timeout silently downgrades to a generic TimeoutException and every truncated response to a generic error — defeating the typed error hierarchy callers rely on. The is_incomplete_message() downcast used just above shows the robust approach is available; it just isn't used consistently.

Suggested direction: classify via typed downcasts / is_* accessors on the error chain instead of stringly-matching Debug output.