auth: thread provider extra_headers into request headers (C6)

Add provider.mergeHeaders and use it in both the openai_chat and
anthropic_messages providers so config extra_headers ride on the model
request, merged onto the built-in header set.

1 files changed, 11 insertions, 2 deletions

diff --git a/libpanto/src/provider_openai_chat.zig b/libpanto/src/provider_openai_chat.zig
index 5713b5d..b236a60 100644
--- a/libpanto/src/provider_openai_chat.zig
+++ b/libpanto/src/provider_openai_chat.zig
@@ -103,18 +103,27 @@ pub const OpenAIChatRequest = struct {
         );
         defer self.allocator.free(auth_value);
 
-        const extra_headers = [_]http.Header{
+        const base_headers = [_]http.Header{
             .{ .name = "content-type", .value = "application/json" },
             .{ .name = "accept", .value = "text/event-stream" },
             .{ .name = "authorization", .value = auth_value },
         };
+        // Merge provider `extra_headers` (e.g. Copilot editor identity, or
+        // auth-exchange-derived headers) onto the base set. Freed at the end
+        // of `open` — after the request body has been flushed.
+        const extra_headers = try provider_mod.mergeHeaders(
+            self.allocator,
+            &base_headers,
+            self.config.extra_headers,
+        );
+        defer self.allocator.free(extra_headers);
 
         // Open the request. We can't use `fetch()` because it buffers the
         // response; we want to stream the body as it arrives. The request
         // is moved into the heap struct so the body reader (which borrows
         // `&rr.response`) stays valid across `produce` calls.
         rr.req = try self.http_client.request(.POST, uri, .{
-            .extra_headers = &extra_headers,
+            .extra_headers = extra_headers,
             // Disable compression: gzip buffers small SSE frames, defeating
             // the streaming property we paid for `stream: true` to get.
             .headers = .{ .accept_encoding = .{ .override = "identity" } },