/v1/messages request builder (cache_control breakpoints on the first 2
system blocks + last 2 messages, tool schema -> input_schema, extended
thinking budget) and an SSE decoder built on eventsource-stream, mapping
content_block_start/delta/stop and message_delta into our normalized
LlmEvent stream (text, thinking+signature, streamed tool-call JSON
accumulated and parsed at content_block_stop, usage merged from
message_start + message_delta). AnthropicProvider wires this to reqwest
with x-api-key/anthropic-version/anthropic-beta headers and classifies
HTTP errors into RateLimited/Auth/Overloaded/Http. ProviderRegistry
resolves "provider/model" strings.
Also tightened processor::process_step's cancellation: the event loop now
selects the stream poll against ctx.cancel instead of only checking at the
top of the loop, so a blocked provider stream is actually interrupted by
abort (matches docs/02-engine.md's cancellation semantics).
127 tests passing, clippy clean.
Erik Simon
bbac60d744