inspect: avoid timeout fallback false positives

The lost-sentinel timeout fallback should only classify errors that actually look like formatted context deadline errors. Otherwise an ordinary Wait failure that returns after the inspector deadline fires could be mislabeled as a timeout. Keep errors.Is(DeadlineExceeded) as the primary signal, but require the fallback error text to contain the context deadline message before sampling the per-inspector context.

Add single and multi-agent regressions where Wait returns a normal failure after the deadline has fired; both remain ordinary failures, not timeouts.