Default provider header timeouts to five minutes so slow model startups fail less often

Fixes

• CoreDefault streamed chunk timeouts to five minutes, with false supported to disable them.
• CoreLet Anthropic thinking.blockBinding opt out via config when you need to keep the provider default.@dkindlund
• CoreLimit Anthropic thinking block binding to Claude 5.1+ models so older deployments do not reject requests.
• CoreAvoid unhandled errors when canceling timed-out SSE reads.@AlexanderWillner