Phase 38 fix: default think=false on /v1/chat

Live-test caught the Phase 21 thinking-model trap on first call.
qwen3.5 with max_tokens=50 and default think behavior burned all 50
tokens on hidden reasoning; visible content was "". completion_tokens
exactly matching max_tokens was the tell.

Adapter now defaults think: Some(false) matching scenario.ts hot-path
discipline. Callers that want reasoning (overseers, T3+) opt in via
a non-OpenAI `think: true` extension field on the request.

Verified end-to-end after restart:
- "Lakehouse supports ACID and raw data." (5 words, 516ms)
- "tokio\nasync-std\nsmol" (3 Rust crates, 391ms)
- /v1/usage accumulates across calls (2 req / 95 total tokens)

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

crates/gateway/src/v1/mod.rs

@@ -68,6 +68,14 @@ pub struct ChatRequest {
     /// Phase 39+ wires real streaming.
     #[serde(default)]
     pub stream: Option<bool>,
+    /// Non-OpenAI extension. Passes through to the provider's thinking
+    /// toggle. Default: **false** — hot-path discipline for thinking
+    /// models (qwen3.5, qwen3, gpt-oss) that otherwise burn the token
+    /// budget on hidden reasoning before visible output starts,
+    /// producing empty responses. Set true explicitly when calling an
+    /// overseer / reasoning-heavy path.
+    #[serde(default)]
+    pub think: Option<bool>,
 }
 
 #[derive(Serialize)]