xserv-chat: filter harmony control tokens + stop at <|end|> for gpt-oss

The gpt-oss harmony format generates internal control tokens
(<|channel|>, <|start|>, <|end|>, <|message|>) that should not appear
in the user-facing output. Additionally, <|end|> marks the end of a
response segment but was not in the model's EOS list, causing the
model to self-prompt into analysis channels and loop.

Fix: treat <|end|> as a stop token, skip all harmony special tokens
from the output stream.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/xserv-model/src/bin/xserv-chat.rs

@@ -397,6 +397,7 @@ fn main() {
             max_new_tokens,
             use_color,
             &tp_handle,
+            is_moe,
         );
         match finish {
             Finish::Stop { token_id } => {
@@ -607,7 +608,16 @@ fn generate_with_paged_cache(
     max_tokens: usize,
     use_color: bool,
     tp: &Option<TpHandle>,
+    is_moe: bool,
 ) -> Finish {
+    let harmony_end_id = if is_moe { tokenizer.special_token_id("<|end|>") } else { None };
+    let harmony_special: Vec<u32> = if is_moe {
+        ["<|channel|>", "<|start|>", "<|end|>", "<|message|>", "<|return|>"]
+            .iter().filter_map(|s| tokenizer.special_token_id(s)).collect()
+    } else {
+        Vec::new()
+    };
+
     if let Some(h) = tp { h.send(TpCommand::Prefill { tokens: prompt_tokens.to_vec(), slot: SLOT }); }
     let logits = model.forward_prefill_paged(prompt_tokens, SLOT, cache);
     if let Some(h) = tp { h.wait(); }
@@ -620,7 +630,7 @@ fn generate_with_paged_cache(
         if let Some(h) = tp { h.send(TpCommand::Decode { tokens: vec![next], positions: vec![position], slots: vec![SLOT] }); }
         let logits = model.forward_decode_paged(&[next], &[position], &[SLOT], cache);
         if let Some(h) = tp { h.wait(); }
-        if tokenizer.is_eos(next) {
+        if tokenizer.is_eos(next) || harmony_end_id == Some(next) {
             print_stream_text(
                 &tokenizer.flush_decode_stream(&mut decode_buffer),
                 in_thinking,
@@ -630,6 +640,11 @@ fn generate_with_paged_cache(
             return Finish::Stop { token_id: next };
         }
 
+        if harmony_special.contains(&next) {
+            next = sample(&logits, sampling);
+            continue;
+        }
+
         print_generated_token(
             tokenizer,
             next,