chat : stop qwen3-coder reasoning and content before template separators (#30557)

* chat : strip trailing whitespace from parsed reasoning and content

Assisted-by: Claude Fable 5.1

* chat : stop qwen3-coder reasoning and content before template separators

Assisted-by: Claude

* tests : remove the Qwen parser case from the chat session tests

Assisted-by: Claude

* common : handle partial prefix until match at end of generation

---------

Co-authored-by: Zeshawn Zahid <zzshawnn@gmail.com>
Co-authored-by: Alde Rojas <hello@alde.dev>

common/peg-parser.cpp

@@ -721,8 +721,9 @@ struct parser_executor {
                 return common_peg_parse_result(COMMON_PEG_PARSE_RESULT_SUCCESS, start_pos, pos, {}, std::move(invalid_utf8));
             }
 
-            if (match == common_trie::PARTIAL_MATCH) {
-                // Found a partial match extending to end of input, return everything before it
+            if (match == common_trie::PARTIAL_MATCH && !ctx.is_complete()) {
+                // Found a partial match extending to end of input, return everything before it.
+                // On a complete input the delimiter can no longer form, so the bytes belong to the text
                 return common_peg_parse_result(COMMON_PEG_PARSE_RESULT_SUCCESS, start_pos, pos, {}, std::move(invalid_utf8));
             }