fix(ds4): separate prefilled reasoning from content (#11802)

DS4 appends the opening thinking marker to tokenizer-templated prompts, so generated text begins directly with reasoning bytes. Starting DsmlParser in TEXT therefore puts the reasoning and closing marker in visible content.

Start the parser in THINK for structured chat requests with thinking enabled in both Predict and PredictStream. Keep the default TEXT state for raw prompts and reasoning-off requests, and add incremental regression coverage.

Assisted-by: Codex:gpt-5

Signed-off-by: Claudio Maradonna <git@codeshifter.xyz>

backend/cpp/ds4/dsml_parser.cpp

@@ -92,7 +92,8 @@ std::string json_escape(const std::string &in) {
 
 } // namespace
 
-DsmlParser::DsmlParser() = default;
+DsmlParser::DsmlParser(bool starts_in_thinking)
+    : state_(starts_in_thinking ? State::THINK : State::TEXT) {}
 
 bool DsmlParser::IsInDsmlStructural() const {
     switch (state_) {