perf: improve memory use and cpu usage for OpenAI requests handled by bridge (#21838)

Apply optimizations:
* https://github.com/openai/openai-go/pull/602
* https://github.com/coder/aibridge/pull/160

These reduce CPU time and allocation count for OpenAI `chat/completions`
and `responses` APIs, making the use of OpenAI chat models through AI
Bridge more performant.

In order to test these changes, we add scaletesting support for the
responses API.

cli/exp_scaletest_bridge.go

@@ -49,6 +49,9 @@ Examples:
   # Test OpenAI API through bridge
   coder scaletest bridge --mode bridge --provider openai --concurrent-users 10 --request-count 5 --num-messages 10
 
+  # Test OpenAI Responses API through bridge
+  coder scaletest bridge --mode bridge --provider responses --concurrent-users 10 --request-count 5 --num-messages 10
+
   # Test Anthropic API through bridge
   coder scaletest bridge --mode bridge --provider anthropic --concurrent-users 10 --request-count 5 --num-messages 10
 
@@ -219,9 +222,9 @@ Examples:
 		{
 			Flag:        "provider",
 			Env:         "CODER_SCALETEST_BRIDGE_PROVIDER",
-			Default:     "openai",
+			Required:    true,
 			Description: "API provider to use.",
-			Value:       serpent.EnumOf(&provider, "openai", "anthropic"),
+			Value:       serpent.EnumOf(&provider, "completions", "messages", "responses"),
 		},
 		{
 			Flag:        "request-count",

cli/exp_scaletest_llmmock.go

@@ -62,6 +62,7 @@ func (*RootCmd) scaletestLLMMock() *serpent.Command {
 
 			_, _ = fmt.Fprintf(inv.Stdout, "Mock LLM API server started on %s\n", srv.APIAddress())
 			_, _ = fmt.Fprintf(inv.Stdout, "  OpenAI endpoint: %s/v1/chat/completions\n", srv.APIAddress())
+			_, _ = fmt.Fprintf(inv.Stdout, "  OpenAI responses endpoint: %s/v1/responses\n", srv.APIAddress())
 			_, _ = fmt.Fprintf(inv.Stdout, "  Anthropic endpoint: %s/v1/messages\n", srv.APIAddress())
 
 			<-ctx.Done()