harness-providers: Anthropic codec + provider

/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.

crates/harness-core/src/engine/processor.rs

@@ -574,18 +574,18 @@ pub async fn process_step(
     let mut result = StepResult::Stop;
 
     loop {
-        let cancelled = ctx.cancel.is_cancelled();
-        if cancelled {
-            run.mark_errored(&ProviderError::Cancelled).await;
-            return Ok(StepOutcome {
-                result: StepResult::Stop,
-                message_id: run.assistant.as_ref().map(|m| m.id.clone()),
-                usage,
-                aborted: true,
-            });
-        }
-
-        let item = stream.next().await;
+        let item = tokio::select! {
+            item = stream.next() => item,
+            _ = ctx.cancel.cancelled() => {
+                run.mark_errored(&ProviderError::Cancelled).await;
+                return Ok(StepOutcome {
+                    result: StepResult::Stop,
+                    message_id: run.assistant.as_ref().map(|m| m.id.clone()),
+                    usage,
+                    aborted: true,
+                });
+            }
+        };
         let Some(item) = item else { break };
 
         let event = match item {