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/context.rs

@@ -4,7 +4,7 @@
 // Journal entries are loaded from the memory graph store, not from
 // a flat file — the parse functions are gone.
 
-use crate::agent::api::types::*;
+use crate::agent::api::*;
 use chrono::{DateTime, Utc};
 use serde::{Deserialize, Serialize};
 use tiktoken_rs::CoreBPE;