AstNode fields are now private with read-only accessors. All mutation
goes through ContextState methods (push, set_message, set_score, del)
which guarantee token_ids stays in sync with text on every leaf.
Also fix ResponseParser to use AstNode::tool_call() constructor,
widen parsing module visibility to pub(crate).
Co-Authored-By: Proof of Concept <poc@bcachefs.org>
Move FunctionCall, FunctionCallDelta, ToolCall, ToolCallDelta from
tools/mod.rs to api/types.rs — these are API wire format, not tool
definitions. Re-export from tools for existing callers.
Move parsing.rs to api/parsing.rs — leaked tool call parsing is API
plumbing.
Co-Authored-By: Proof of Concept <poc@bcachefs.org>