feat(inference): capture reasoning_content on responses (WIP, provider side)

Adds a nullable `reasoning` field to InferenceResponse and maps the
llama.cpp / OpenAI-compat `reasoning_content` field into it, so local models
that emit their chain-of-thought have it captured. Provider/model side only —
the emit-site event field (reasoningArtifactId) + CAS storage in the
orchestrator are still pending (tracked in Vikunja Correx #4).

infrastructure/inference/openai_compat/src/main/kotlin/com/correx/infrastructure/inference/openai/OpenAiApiModels.kt

@@ -28,6 +28,7 @@ data class OpenAiChatCompletionRequest(
 data class OpenAiChatMessage(
     val role: String,
     val content: String? = null,
+    @SerialName("reasoning_content") val reasoningContent: String? = null,
     @SerialName("tool_calls") val toolCalls: List<ToolCallRequest> = emptyList(),
     @SerialName("tool_call_id") val toolCallId: String? = null,
 )