feat(openrouter): cap max_tokens per request (OPENROUTER_MAX_TOKENS)

OpenRouter reserves credit against the requested max_tokens, falling
back to the model's full output window when omitted, so expensive
models failed with credit-limit errors even for short replies. Default
the cap to 16384; OPENROUTER_MAX_TOKENS overrides it and 0 disables it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ai/openrouter.rs

@@ -19,6 +19,11 @@ use futures::stream::{BoxStream, StreamExt};
 const DEFAULT_BASE_URL: &str = "https://openrouter.ai/api/v1";
 const DEFAULT_EMBEDDING_MODEL: &str = "openai/text-embedding-3-small";
 const CACHE_DURATION_SECS: u64 = 15 * 60;
+/// Default completion cap sent as `max_tokens`. OpenRouter pre-authorizes
+/// credit against the requested max (defaulting to the model's full output
+/// window when omitted), so expensive models fail with credit-limit errors
+/// even when the actual reply is short. Override via `OPENROUTER_MAX_TOKENS`.
+pub const DEFAULT_MAX_TOKENS: u32 = 16384;
 
 #[derive(Clone)]
 struct CachedEntry<T> {
@@ -64,6 +69,9 @@ pub struct OpenRouterClient {
     top_p: Option<f32>,
     top_k: Option<i32>,
     min_p: Option<f32>,
+    /// Upper bound on generated tokens per call, sent as `max_tokens`.
+    /// `None` omits the field (provider default).
+    max_tokens: Option<u32>,
     /// Optional `HTTP-Referer` header OpenRouter uses for attribution.
     pub referer: Option<String>,
     /// Optional `X-Title` header OpenRouter uses for attribution.
@@ -87,6 +95,7 @@ impl OpenRouterClient {
             top_p: None,
             top_k: None,
             min_p: None,
+            max_tokens: Some(DEFAULT_MAX_TOKENS),
             referer: None,
             app_title: None,
         }
@@ -115,6 +124,10 @@ impl OpenRouterClient {
         self.min_p = min_p;
     }
 
+    pub fn set_max_tokens(&mut self, max_tokens: Option<u32>) {
+        self.max_tokens = max_tokens;
+    }
+
     pub fn set_attribution(&mut self, referer: Option<String>, app_title: Option<String>) {
         self.referer = referer;
         self.app_title = app_title;
@@ -288,6 +301,9 @@ impl OpenRouterClient {
             // directly transferable. Skip rather than silently mis-map.
             let _ = c;
         }
+        if let Some(m) = self.max_tokens {
+            v.push(("max_tokens", json!(m)));
+        }
         v
     }
 }