harness-providers: OpenAI chat + responses codecs, provider, registry wiring (M3)

- codec/openai_chat.rs: /chat/completions request builder (flat messages,
  tool_calls, system → leading system message, reasoning_effort) + SSE decoder
  (content/tool_calls-by-index/reasoning_content accumulation, [DONE] sentinel,
  include_usage). Also the fallback codec for OpenAI-compatible endpoints.
- codec/openai_responses.rs: /responses request builder (input items, tool calls
  and results as top-level function_call/function_call_output, instructions,
  reasoning {effort, summary}) + SSE decoder (output_item add/done, output_text
  and reasoning_summary deltas, function_call_arguments, response.completed usage
  with separate reasoning_tokens).
- openai.rs: OpenAiProvider routing gpt-*/o-* → responses, else chat; Bearer auth;
  error classification incl. 400 context_length_exceeded → ContextOverflow.
- Registered in harness-app from config.providers["openai"] (api_key + optional
  base_url), sourced from OPENAI_API_KEY by the existing config loader.
- 21 codec/provider unit tests (decode fixtures + request-builder assertions).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/harness-app/src/lib.rs

@@ -19,7 +19,7 @@ use harness_core::tool::ToolRegistry;
 use harness_core::types::{
     Message, MessageId, ModelRef, Part, PartBody, PartId, Session, SessionId,
 };
-use harness_providers::{AnthropicProvider, ProviderRegistry};
+use harness_providers::{AnthropicProvider, OpenAiProvider, ProviderRegistry};
 use tokio::task::JoinHandle;
 use tokio_util::sync::CancellationToken;
 
@@ -122,6 +122,21 @@ impl EngineHandle {
         {
             providers.register(Arc::new(AnthropicProvider::new(key)));
         }
+        if let Some(key) = config
+            .providers
+            .get("openai")
+            .and_then(|p| p.api_key.clone())
+        {
+            let provider = match config
+                .providers
+                .get("openai")
+                .and_then(|p| p.base_url.clone())
+            {
+                Some(base_url) => OpenAiProvider::with_base_url(key, base_url),
+                None => OpenAiProvider::new(key),
+            };
+            providers.register(Arc::new(provider));
+        }
 
         let data_dir = dirs::data_dir()
             .unwrap_or_else(std::env::temp_dir)