v1: accept OpenAI multimodal content shape (array-of-parts)

Modern OpenAI clients (pi-ai, openai SDK 6.x, langchain-js, the official
agents) send `messages[].content` as an array of content parts:
`[{type:"text", text:"..."}, {type:"image_url", ...}]`. Our gateway
typed `content` as plain `String` and 422'd those calls.

Fix: `Message.content` is now `serde_json::Value` so requests
deserialize regardless of shape. `Message::text()` flattens
content-parts arrays (concat'd `text` fields, non-text parts skipped)
for places that need a plain string — Ollama prompt assembly, char
counts, the assistant's own response synthesis. `Message::new_text()`
constructs string-content messages without writing the wrapper at
each call site. Forwarders (openrouter) clone content through
verbatim so providers see exactly what the client sent.

Verified end-to-end: Pi CLI (`pi --print --provider openrouter`)
landed a clean 1902-token request through `/v1/chat/completions`,
routed to OpenRouter as `openai/gpt-oss-120b:free`, response in
1.62s, Langfuse trace `v1.chat:openrouter` recorded with provider
tag. Same path that any tool using the official openai SDK takes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/gateway/src/v1/ollama.rs

@@ -60,10 +60,7 @@ pub async fn chat(client: &AiClient, req: &ChatRequest) -> Result<ChatResponse,
         model: resp.model,
         choices: vec![Choice {
             index: 0,
-            message: Message {
-                role: "assistant".into(),
-                content: resp.text,
-            },
+            message: Message::new_text("assistant", resp.text),
             finish_reason: "stop".into(),
         }],
         usage: UsageBlock {
@@ -89,13 +86,14 @@ fn flatten_messages(messages: &[Message]) -> (String, String) {
     let mut system = String::new();
     let mut prompt = String::new();
     for m in messages {
+        let body = m.text();
         if m.role == "system" {
             if !system.is_empty() { system.push('\n'); }
-            system.push_str(&m.content);
+            system.push_str(&body);
         } else {
             prompt.push_str(&m.role);
             prompt.push_str(": ");
-            prompt.push_str(&m.content);
+            prompt.push_str(&body);
             prompt.push_str("\n\n");
         }
     }
@@ -104,7 +102,7 @@ fn flatten_messages(messages: &[Message]) -> (String, String) {
 }
 
 fn estimate_prompt_tokens(messages: &[Message]) -> u32 {
-    let chars: usize = messages.iter().map(|m| m.content.chars().count()).sum();
+    let chars: usize = messages.iter().map(|m| m.text().chars().count()).sum();
     ((chars + 3) / 4) as u32
 }