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, 9 insertions, 1 deletions

diff --git a/libpanto/src/provider_anthropic_messages.zig b/libpanto/src/provider_anthropic_messages.zig
index be4268e..8f5d200 100644
--- a/libpanto/src/provider_anthropic_messages.zig
+++ b/libpanto/src/provider_anthropic_messages.zig
@@ -96,7 +96,15 @@ pub const AnthropicMessagesRequest = struct {
                 .value = "interleaved-thinking-2025-05-14",
             };
         }
-        const extra_headers = headers_buf[0 .. if (send_interleaved) @as(usize, 5) else @as(usize, 4)];
+        const base_headers = headers_buf[0 .. if (send_interleaved) @as(usize, 5) else @as(usize, 4)];
+        // Merge any provider `extra_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);
 
         rr.req = try self.http_client.request(.POST, uri, .{
             .extra_headers = extra_headers,