Kill dead API types: ChatRequest, ChatCompletionChunk, Delta, streaming types

Removed all chat completions wire types that are no longer used:
ChatRequest, ReasoningConfig, ChatCompletionChunk, ChunkChoice,
Delta, FunctionCallDelta, ToolCallDelta, append_content, user_with_images.

Remaining types in api/types.rs are transitional (Message, ToolCall, etc.)
— they'll go away as outer callers migrate to AstNode.

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

src/agent/api/mod.rs

@@ -10,7 +10,7 @@ pub mod http;
 mod types;
 mod openai;
 
-// Public API types — used outside agent::api
+// Transitional — these will go away as callers migrate to AstNode
 pub use types::{Message, MessageContent, ContentPart, ImageUrl, Role, ToolCall, FunctionCall, Usage};
 
 use anyhow::Result;