fix(stream): robust reasoning/content split + flush un-tagged output

The incremental </think> search missed tags split across tokens, inverting
reasoning/content classification. Detect the first </think> on the full
buffer and track the content boundary as a byte offset. If the model never
closes </think>, flush the buffered text as content so clients always
receive the response. Chat UI now renders reasoning_content too.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/application/chat/mod.rs

@@ -1,6 +1,6 @@
 pub mod use_cases;
 
 pub use use_cases::{
-    build_prompt, build_sampler, clean_text, parse_tool_calls, split_stream_chunk, validate_model,
-    SamplerParams,
+    build_prompt, build_sampler, clean_text, parse_tool_calls, split_stream_chunk, strip_markup,
+    validate_model, SamplerParams,
 };