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/mod.rs

@@ -23,7 +23,7 @@ use anyhow::Result;
 use tiktoken_rs::CoreBPE;
 
 use api::{ApiClient, ToolCall};
-use api::types::{ContentPart, Message, MessageContent, Role};
+use api::{ContentPart, Message, MessageContent, Role};
 use context::{ConversationEntry, ContextState};
 use tools::{summarize_args, working_stack};