chore(neuron): bump default max_tokens from 512 to 8192

512 is too low for any modern coding model — clients that don't
explicitly set max_tokens get clipped responses with no diagnostic.
Bump the fallback at all four inference call sites (single-GPU
streaming + non-streaming, TP leader + non-leader) to 8192, which
fits comfortably within Qwen3-class context windows after a
typical agent prompt and lines up with what helexa-acp / a0 / curl
clients reasonably expect.

Clients that explicitly set max_tokens (now including helexa-acp
via HELEXA_ACP_MAX_TOKENS / per-endpoint TOML) override this.

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

crates/neuron/src/harness/candle.rs

@@ -1381,7 +1381,7 @@ impl CandleHarness {
 
             let temperature = request.temperature.unwrap_or(0.7);
             let top_p = request.top_p;
-            let max_new = request.max_tokens.unwrap_or(512) as usize;
+            let max_new = request.max_tokens.unwrap_or(8192) as usize;
             let seed = unix_subsec_nanos();
 
             let eos_id = loaded
@@ -1620,7 +1620,7 @@ impl CandleHarness {
 
         let temperature = request.temperature.unwrap_or(0.7);
         let top_p = request.top_p;
-        let max_new = request.max_tokens.unwrap_or(512) as usize;
+        let max_new = request.max_tokens.unwrap_or(8192) as usize;
         let seed = unix_subsec_nanos();
 
         let eos_id = loaded
@@ -2264,7 +2264,7 @@ impl CandleHarness {
 
         let temperature = request.temperature.unwrap_or(0.7);
         let top_p = request.top_p;
-        let max_new = request.max_tokens.unwrap_or(512) as usize;
+        let max_new = request.max_tokens.unwrap_or(8192) as usize;
         let seed = unix_subsec_nanos();
 
         let eos_id = tp
@@ -2598,7 +2598,7 @@ async fn chat_completion_tp_inner(
 
     let temperature = request.temperature.unwrap_or(0.7);
     let top_p = request.top_p;
-    let max_new = request.max_tokens.unwrap_or(512) as usize;
+    let max_new = request.max_tokens.unwrap_or(8192) as usize;
     let seed = unix_subsec_nanos();
 
     let eos_id = tp