Redesign context AST: typed NodeBody, Role as grammar roles, tests

Role is now just System/User/Assistant — maps 1:1 to the grammar.
Leaf types are NodeBody variants: Content, Thinking, ToolCall,
ToolResult, Memory, Dmn, Log. Each variant renders itself; no Role
needed on leaves. AstNode is Leaf(NodeLeaf) | Branch{role, children}.
ContextState holds four Vec<AstNode> sections directly.

Moved tool call XML parsing from api/parsing.rs into context_new.rs
so all grammar knowledge lives in one place.

Tokenizer encode() now returns empty vec when uninitialized instead
of panicking, so tests work without the tokenizer file.

26 tests: XML parsing, incremental streaming (char-by-char feeds
found and fixed a lookahead bug), rendering for all node types,
tokenizer round-trip verification.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/tokenizer.rs

@@ -25,17 +25,21 @@ pub fn init(path: &str) {
     TOKENIZER.set(t).ok();
 }
 
-/// Get the global tokenizer. Panics if not initialized.
-fn get() -> &'static Tokenizer {
-    TOKENIZER.get().expect("tokenizer not initialized — call tokenizer::init() first")
+/// Get the global tokenizer. Returns None if not initialized.
+fn get() -> Option<&'static Tokenizer> {
+    TOKENIZER.get()
 }
 
 /// Tokenize a raw string, returning token IDs.
+/// Returns empty vec if the tokenizer is not initialized.
 pub fn encode(text: &str) -> Vec<u32> {
-    get().encode(text, false)
+    match get() {
+        Some(t) => t.encode(text, false)
             .unwrap_or_else(|e| panic!("tokenization failed: {}", e))
             .get_ids()
-        .to_vec()
+            .to_vec(),
+        None => vec![],
+    }
 }
 
 /// Tokenize a chat entry with template wrapping:
@@ -59,8 +63,11 @@ pub fn count(text: &str) -> usize {
 
 /// Decode token IDs back to text.
 pub fn decode(ids: &[u32]) -> String {
-    get().decode(ids, true)
-        .unwrap_or_else(|e| panic!("detokenization failed: {}", e))
+    match get() {
+        Some(t) => t.decode(ids, true)
+            .unwrap_or_else(|e| panic!("detokenization failed: {}", e)),
+        None => String::new(),
+    }
 }
 
 /// Check if the tokenizer is initialized.