fix(ai): skip errored/aborted assistant messages in transform-messages

Fixes OpenAI Responses 400 error 'reasoning without following item' by
skipping errored/aborted assistant messages entirely rather than filtering
at the provider level. This covers openai-responses, openai-codex-responses,
and future providers.

Removes strictResponsesPairing compat option (no longer needed).

Closes #838

packages/coding-agent/src/core/model-registry.ts

@@ -28,7 +28,7 @@ const OpenAICompletionsCompatSchema = Type.Object({
 });
 
 const OpenAIResponsesCompatSchema = Type.Object({
-	strictResponsesPairing: Type.Optional(Type.Boolean()),
+	// Reserved for future use
 });
 
 const OpenAICompatSchema = Type.Union([OpenAICompletionsCompatSchema, OpenAIResponsesCompatSchema]);