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-providers/Cargo.toml

@@ -6,6 +6,21 @@ license.workspace = true
 
 [dependencies]
 harness-core = { workspace = true }
+tokio = { workspace = true }
+tokio-util = { workspace = true }
+futures = { workspace = true }
+async-stream = { workspace = true }
+async-trait = { workspace = true }
+reqwest = { workspace = true }
+eventsource-stream = { workspace = true }
+bytes = { workspace = true }
+serde = { workspace = true }
+serde_json = { workspace = true }
+thiserror = { workspace = true }
+tracing = { workspace = true }
+
+[dev-dependencies]
+tokio = { workspace = true, features = ["test-util", "macros"] }
 
 [lints]
 workspace = true