Accumulate streamed tool calls across chunks in Ollama streaming

Ollama >=0.8 can stream tool_calls incrementally across NDJSON chunks;
chat_with_tools_stream did `tool_calls = Some(tcs)` per chunk, so only
the last chunk's calls survived assembly and earlier calls were silently
dropped. Append into the accumulator instead.

- ollama: append_streamed_tool_calls helper + tests covering two calls
  arriving in separate chunks and the single-chunk batch case.
- llamacpp: the SSE delta assembly was already correct (per-index
  BTreeMap, same-index argument fragments concatenate, distinct indexes
  accumulate); extracted it into apply_tool_call_deltas /
  finalize_tool_calls and added tests pinning that behavior.
- llm_client: new shared strip_think_blocks (moved from ollama's private
  extract_final_answer, which now delegates) so the tool-calling final
  content paths can reuse it; unit tests for tagged/plain/unclosed/empty
  cases.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/ai/llm_client.rs

@@ -170,3 +170,55 @@ pub struct ModelCapabilities {
     pub has_vision: bool,
     pub has_tool_calling: bool,
 }
+
+/// Strip a leading `<think>…</think>` reasoning block from model output.
+///
+/// Thinking models sometimes emit chain-of-thought inside think tags before
+/// the real answer. Everything after the first `</think>` is the answer;
+/// when no tag is present — or the text after it is empty — the trimmed
+/// input is returned unchanged. Mirrors the behavior Ollama's
+/// `extract_final_answer` has applied to single-shot generation; shared here
+/// so the tool-calling final-content paths (agentic generation + chat) can
+/// apply the identical cleanup before parsing / persisting.
+pub fn strip_think_blocks(response: &str) -> String {
+    let response = response.trim();
+
+    if let Some(pos) = response.find("</think>") {
+        let answer = response[pos + "</think>".len()..].trim();
+        if !answer.is_empty() {
+            return answer.to_string();
+        }
+    }
+
+    response.to_string()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn strip_think_blocks_removes_leading_think_block() {
+        let raw = "<think>\nLet me reason about this.\n</think>\n\nTitle: A Day Out\n\nThe body.";
+        assert_eq!(strip_think_blocks(raw), "Title: A Day Out\n\nThe body.");
+    }
+
+    #[test]
+    fn strip_think_blocks_passes_through_plain_content() {
+        assert_eq!(strip_think_blocks("  just an answer  "), "just an answer");
+    }
+
+    #[test]
+    fn strip_think_blocks_keeps_content_when_answer_after_tag_is_empty() {
+        // A think block with nothing after it: better to return the trimmed
+        // original than an empty string (matches Ollama's fallback).
+        let raw = "<think>only thoughts</think>";
+        assert_eq!(strip_think_blocks(raw), raw);
+    }
+
+    #[test]
+    fn strip_think_blocks_handles_unclosed_tag() {
+        let raw = "<think>thinking forever";
+        assert_eq!(strip_think_blocks(raw), raw);
+    }
+}