auth: openai_responses provider for Codex subscription (Tier 2)

Add the OpenAI Responses API wire dialect (openai_responses APIStyle +
OpenAIResponsesConfig), a request serializer (instructions/input items,
flat function tools, reasoning, store:false, include encrypted reasoning),
and a streaming state machine over the typed response.* SSE events mapping
to the shared block model. Wire dispatch, buildProviderConfig, the C-ABI
free switch, and the reasoning selectors. Ship a commented Codex example in
the default config.

Implemented from the Responses API docs + the open-source Codex client and
covered by fixture tests; live verification against a ChatGPT plan is still
required (documented).

1 files changed, 8 insertions, 0 deletions

diff --git a/src/config_file.zig b/src/config_file.zig
index 1277661..cac003a 100644
--- a/src/config_file.zig
+++ b/src/config_file.zig
@@ -196,6 +196,14 @@ pub fn buildProviderConfig(
             .prompt_cache = prov.prompt_cache,
             .extra_headers = prov.extra_headers,
         } },
+        .openai_responses => return .{ .openai_responses = .{
+            .api_key = api_key,
+            .base_url = prov.base_url,
+            .model = wire_model,
+            .reasoning = reasoning,
+            .max_tokens = max_tokens,
+            .extra_headers = prov.extra_headers,
+        } },
     }
 }