feat: render GGUF Jinja template via minijinja crate

- Replaced manual prompt building with GGUF's chat template rendered through minijinja (Rust Jinja2 engine)
- Simplified template: removed multi-step tool detection, reasoning extraction (not needed at template level)
- Added reasoning_content separation: clean_text() returns (reasoning, answer) tuple
- Added reasoning_content field to ResponseMessage and SseDelta for OpenAI-compatible output
- Embedded template at build time via include_str! from templates/chat_template.jinja
- Built-in support for enable_thinking, tool_definitions, tool_calls, tool_response

Cargo.lock

@@ -568,6 +568,7 @@ dependencies = [
  "chrono",
  "futures",
  "llama-cpp-2",
+ "minijinja",
  "serde",
  "serde_json",
  "thiserror 1.0.69",
@@ -615,12 +616,28 @@ version = "2.8.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "cf8baf1c55e62ffcace7a9f06f4bd9cd3f0c4beb022d3b367256b91b87513d98"
 
+[[package]]
+name = "memo-map"
+version = "0.3.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "38d1115007560874e373613744c6fba374c17688327a71c1476d1a5954cc857b"
+
 [[package]]
 name = "mime"
 version = "0.3.17"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "6877bb514081ee2a7ff5ef9de3281f14a4dd4bceac4c09388074a6b5df8a139a"
 
+[[package]]
+name = "minijinja"
+version = "2.21.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cb3d648e68cea56d9858d535ee28f9538404e2dd8cb08ed0bd05dca379477f39"
+dependencies = [
+ "memo-map",
+ "serde",
+]
+
 [[package]]
 name = "minimal-lexical"
 version = "0.2.1"