Restrict API types visibility — types module is now private

Only Message, Role, MessageContent, ContentPart, ToolCall,
FunctionCall, Usage, ImageUrl are pub-exported from agent::api.

Internal types (ChatRequest, ChatCompletionChunk, ChunkChoice,
Delta, ReasoningConfig, ToolCallDelta, FunctionCallDelta) are
pub(crate) — invisible outside the crate.

All callers updated to import from agent::api:: instead of
agent::api::types::.

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

src/agent/oneshot.rs

@@ -14,8 +14,7 @@ use std::fs;
 use std::path::PathBuf;
 use std::sync::OnceLock;
 
-use super::api::ApiClient;
-use super::api::types::*;
+use super::api::{self, ApiClient, Message, Usage};
 use super::tools::{self as agent_tools};
 use super::Agent;